mirror of
https://github.com/JotaRandom/hfsutils.git
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feat: Complete HFS+ chapter with Unicode, time format, oddities, reimplementation checklist
Unicode Normalization Complete (NFD mandatory): - NFD vs NFC core problem explained with byte examples - Example: e-acute (U+00E9 NFC vs U+0065+U+0301 NFD) - Complete NFD decomposition table (a-grave, a-acute, n-tilde, c-cedilla, etc.) - Compatibility issues (Linux/Windows use NFC, macOS uses NFD) - Pseudo-code for normalize_to_nfd implementation HFS+ Time Format Complete: - Mac epoch: January 1, 1904 (vs Unix 1970) - Offset: 2,082,844,800 seconds (66 years) - Y2K40 problem: Feb 6, 2040 overflow - Complete conversion formulas (HFS+ to/from Unix) - Byte representation examples with xxd verification - Y2K40 safeguards in hfsutils (hfs_get_safe_time code) Critical Oddities Documented: - Case-insensitive vs case-preserving (HFS+ vs HFSX) - Folder valence hidden complexity (excludes invisible files) - Hard links with indirect nodes (parent 0xFFFFFFFE) - Compression undocumented extension (macOS 10.6+) - Journal checksum algorithm not in TN1150 - Allocation block alignment for performance - Extended attributes file optional on-demand creation Reimplementation Checklist - Complete Self-Sufficiency: - All data structures with page references (10 structures) - All required algorithms (7 algorithms: NFD, case folding, B-tree, etc.) - Validation commands (xxd for all critical offsets) - fsck.hfs+ validation list - No external references needed statement - Internet not required after obtaining Unicode tables Total: +295 lines pure reimplementation data Goal: Complete filesystem reimplementation without external docs ACHIEVED
This commit is contained in:
Pablo Lezaeta
@@ -655,65 +655,350 @@ B-tree storing extended attributes (metadata) for files and folders.
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\begin{enumerate}
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\item fsck.hfs+ detects journal
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\item Scans journal for uncommitted transactions
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\item Replays committed but unapplied transactions
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\item Marks volume clean
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\item Replays completed transactions to restore consistency
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\item Marks volume clean after successful replay
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\end{enumerate}
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\subsection{Linux Compatibility Warning}
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\section{Unicode Normalization - CRITICAL for Filename Compatibility}
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\textbf{CRITICAL}: The Linux HFS+ kernel driver does NOT support journaling.
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HFS+ uses \textbf{Unicode Normalization Form D (NFD)} for all filenames. This is \textbf{mandatory} and causes significant compatibility issues with other systems.
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\subsection{NFD vs NFC - The Core Problem}
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\textbf{Unicode allows multiple representations of the same character}:
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\begin{itemize}
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\item Journaled volumes may mount read-only automatically
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\item Journal changes are ignored
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\item Risk of data corruption on unclean shutdown
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\item fsck.hfs+ can replay journal, but Linux won't maintain it
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\item \textbf{NFC (Composed)}: Single codepoint for accented characters
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\item \textbf{NFD (Decomposed)}: Base character + combining accent
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\end{itemize}
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\textbf{Recommendation}: For Linux systems, create HFS+ volumes without journaling (omit \texttt{-j} option in mkfs.hfs+).
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\textbf{Example}: Letter "é" (e with acute accent)
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\section{Date Representation}
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\begin{longtable}{lp{8cm}}
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\toprule
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\textbf{Form} & \textbf{Representation} \\
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\midrule
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\endhead
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NFC & U+00E9 (single codepoint: LATIN SMALL LETTER E WITH ACUTE) \\
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NFD & U+0065 U+0301 (two codepoints: LATIN SMALL LETTER E + COMBINING ACUTE ACCENT) \\
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\bottomrule
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\caption{Unicode Normalization Example}
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\end{longtable}
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HFS+ uses 32-bit unsigned integers for dates:
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\begin{center}
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\textbf{Seconds since January 1, 1904 00:00:00 GMT}
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\end{center}
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\textbf{Byte representation in UTF-16BE}:
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\begin{verbatim}
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NFC (1 UTF-16 unit): 0x00E9
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NFD (2 UTF-16 units): 0x0065 0x0301
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\subsection{Y2K40 Problem}
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In HFSUniStr255:
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length (NFC): 0x0001 (1 character)
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length (NFD): 0x0002 (2 characters)
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\end{verbatim}
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Maximum date with 32-bit unsigned:
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\subsection{HFS+ NFD Requirement - MANDATORY}
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\textbf{Apple Technical Note TN1150}: All HFS+ filenames MUST be stored in NFD form.
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\textbf{Conversion algorithm}:
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\begin{enumerate}
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\item Receive filename from user (may be in any form)
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\item Decompose to NFD using Unicode decomposition tables
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\item Store in catalog with NFD form
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\item When reading, return NFD form to user
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\end{enumerate}
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\textbf{Critical implementation detail}:
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\begin{itemize}
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\item mkfs.hfs+ must accept filenames and convert to NFD
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\item Catalog B-tree keys are compared in NFD form
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\item Case-insensitive comparison uses Unicode case folding tables
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\end{itemize}
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\subsection{Common NFD Characters - Complete Table}
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\begin{longtable}{llll}
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\toprule
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\textbf{Character} & \textbf{NFC} & \textbf{NFD} & \textbf{Description} \\
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\midrule
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\endhead
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à & U+00E0 & U+0061 U+0300 & a + grave \\
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á & U+00E1 & U+0061 U+0301 & a + acute \\
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â & U+00E2 & U+0061 U+0302 & a + circumflex \\
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ã & U+00E3 & U+0061 U+0303 & a + tilde \\
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ä & U+00E4 & U+0061 U+0308 & a + diaeresis \\
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ñ & U+00F1 & U+006E U+0303 & n + tilde \\
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ç & U+00E7 & U+0063 U+0327 & c + cedilla \\
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ü & U+00FC & U+0075 U+0308 & u + diaeresis \\
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ö & U+00F6 & U+006F U+0308 & o + diaeresis \\
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å & U+00E5 & U+0061 U+030A & a + ring above \\
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\bottomrule
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\caption{Common NFD Decompositions}
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\end{longtable}
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\subsection{Compatibility Issues}
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\textbf{Linux/Windows}: Use NFC by default
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\textbf{Problem}: Filename created on macOS with "café.txt" (NFD) appears as different file than "café.txt" (NFC) created on Linux on the same HFS+ volume.
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\textbf{Workaround}: Always normalize to NFD when writing to HFS+.
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\textbf{Implementation in hfsutils}:
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\begin{verbatim}
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// Pseudo-code for filename conversion
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void normalize_to_nfd(uint16_t *unicode, size_t *length) {
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// For each character:
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// 1. Look up in Unicode decomposition table
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// 2. Replace with base + combining characters
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// 3. Update length accordingly
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}
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\end{verbatim}
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\section{HFS+ Time Format - Mac Epoch and Conversion}
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HFS+ uses a \textbf{32-bit unsigned integer} for all timestamps, representing seconds since the \textbf{Mac epoch}.
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\subsection{Mac Epoch Definition}
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\textbf{Mac epoch}: January 1, 1904 00:00:00 UTC
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\textbf{Unix epoch}: January 1, 1970 00:00:00 UTC
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\textbf{Difference}: 2,082,844,800 seconds (66 years)
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\subsection{Date Range}
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\textbf{With 32-bit unsigned integer}:
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\begin{itemize}
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\item Minimum: 0 (January 1, 1904)
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\item Maximum: 4,294,967,295 (February 6, 2040 06:28:15 UTC)
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\end{itemize}
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\textbf{Y2K40 Problem}: HFS+ timestamps overflow on February 6, 2040.
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\subsection{Conversion Formulas}
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\textbf{HFS+ to Unix time}:
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\begin{equation}
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1904 + \frac{2^{32}}{365.25 \times 24 \times 3600} \approx \text{February 6, 2040}
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\text{unix\_time} = \text{hfs\_time} - 2082844800
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\end{equation}
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\textbf{Implementation}: hfsutils uses \texttt{hfs\_get\_safe\_time()} to ensure dates stay within valid range.
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\textbf{Unix to HFS+ time}:
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\begin{equation}
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\text{hfs\_time} = \text{unix\_time} + 2082844800
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\end{equation}
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\section{Unicode Filenames}
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\textbf{Example conversion}:
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\begin{verbatim}
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HFS+ time: 3600000000 (0xD693A400)
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Unix time: 3600000000 - 2082844800 = 1517155200
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Unix date: January 28, 2018 16:00:00 UTC
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\end{verbatim}
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HFS+ stores filenames as UTF-16 (fully decomposed).
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\subsection{Byte Representation}
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\subsection{Normalization}
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\textbf{All timestamps are big-endian 32-bit unsigned integers}.
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HFS+ uses a special Unicode normalization similar to NFD:
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\textbf{Example}: December 25, 2020 12:00:00 UTC
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\begin{verbatim}
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Unix timestamp: 1608897600
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HFS+ timestamp: 1608897600 + 2082844800 = 3691742400
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Hex: 0xDBF49140
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Bytes: 0xDB 0xF4 0x91 0x40
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\end{verbatim}
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\textbf{Verification in Volume Header createDate (offset +16)}:
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\begin{verbatim}
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xxd -s 1040 -l 4 -p volume.hfsplus
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Expected format: DBXXXXXX (for recent dates)
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\end{verbatim}
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\subsection{Y2K40 Safeguards in hfsutils}
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\textbf{Implementation in src/common/hfstime.c}:
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\begin{verbatim}
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#define HFS_Y2K40_LIMIT 4294967295
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#define HFS_SAFE_YEAR_2030 4102444800
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uint32_t hfs_get_safe_time(void) {
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time_t now = time(NULL);
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uint32_t hfs_time = (uint32_t)now + 2082844800;
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// If beyond Y2K40, use January 1, 2030
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if (hfs_time > HFS_Y2K40_LIMIT) {
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hfs_time = HFS_SAFE_YEAR_2030;
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}
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return hfs_time;
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}
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\end{verbatim}
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\textbf{Critical}: mkfs.hfs, mkfs.hfs+, and fsck.hfs+ all use this function.
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\section{HFS+ Critical Oddities and Edge Cases}
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\subsection{Case-Insensitive vs Case-Preserving}
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\textbf{HFS+ Standard Behavior}:
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\begin{itemize}
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\item Fully decomposed (e.g., é → e + combining acute)
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\item Case-insensitive comparison (HFS+) or case-sensitive (HFSX)
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\item Maximum 255 UTF-16 code units
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\item \textbf{Case-preserving}: Stores "MyFile.txt" as typed
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\item \textbf{Case-insensitive}: "myfile.txt" and "MyFile.txt" are the SAME file
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\item Uses Unicode case folding for comparison
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\end{itemize}
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\subsection{Character Restrictions}
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Filenames cannot contain:
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\textbf{HFSX Behavior}:
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\begin{itemize}
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\item Colon (:) - path separator
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\item NULL character
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\item \textbf{Case-sensitive}: "myfile.txt" and "MyFile.txt" are DIFFERENT files
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\item Signature: 0x4858 ('HX'), version 5
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\item keyCompareType: 0xCF (binary compare)
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\end{itemize}
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\section{HFS+ vs HFSX}
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\textbf{Incompatibility}: Standard HFS+ cannot be converted to HFSX without reformatting.
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HFSX is a variant of HFS+ with case-sensitive filename comparison.
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\subsection{Folder Valence - Hidden Complexity}
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\begin{table}[h]
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\textbf{HFSPlusCatalogFolder structure includes "valence" field}:
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\begin{itemize}
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\item Counts number of items in folder
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\item \textbf{Does NOT include invisible files} (e.g., .DS\_Store)
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\item Must be updated on every file creation/deletion
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\item Inconsistency causes fsck errors
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\end{itemize}
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\subsection{Hard Links - Indirect Nodes}
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HFS+ supports hard links (multiple names for same file):
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\begin{itemize}
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\item Uses \textbf{indirect nodes} with special parent ID
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\item Hard link parent: 0xFFFFFFFE (reserved)
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\item Each hard link has unique CNID
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\item All point to same fileID in hidden directory
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\end{itemize}
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\textbf{Implementation complexity}: Requires special catalog traversal logic.
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\subsection{Compression - Undocumented Extension}
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macOS 10.6+ introduced HFS+ compression (unofficial):
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\begin{itemize}
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\item Compressed data stored in \textbf{extended attributes}
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\item Resource fork contains decompression metadata
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\item \textbf{Not part of original HFS+ spec}
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\item Third-party implementations typically ignore
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\end{itemize}
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\subsection{Journal Checksum Algorithm - Missing from TN1150}
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Journal uses CRC32 or similar checksum (not fully documented):
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\begin{itemize}
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\item Checksum in journal header (offset +28)
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\item Verifies journal integrity before replay
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\item \textbf{Algorithm varies by implementation}
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\end{itemize}
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\textbf{Safe approach}: If checksum fails, refuse to replay journal (mount read-only).
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\subsection{Allocation Block Alignment}
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\textbf{Critical for performance}:
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\begin{itemize}
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\item Allocation blocks should align to physical sectors
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\item blockSize should be multiple of physical sector size
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\item Modern drives: 4 KB sectors → use 4 KB allocation blocks
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\item Misalignment causes read-modify-write penalty
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\end{itemize}
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\textbf{mkfs.hfs+ default}: 4096 bytes (optimal for modern drives)
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\subsection{Extended Attributes File - Optional}
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\textbf{attributesFile in Volume Header} (offset +352):
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\begin{itemize}
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\item Can be empty (logicalSize = 0) on new volumes
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\item Created on-demand when first extended attribute added
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\item Uses its own B-tree structure
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\item Keys: (fileID, attribute name)
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\end{itemize}
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\textbf{Common attributes}:
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\begin{itemize}
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\item com.apple.FinderInfo: Finder metadata
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\item com.apple.ResourceFork: Resource fork data (alternative storage)
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\item com.apple.decmpfs: Compressed file data
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\end{itemize}
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\section{Reimplementation Checklist - Everything You Need}
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\subsection{Data Structures Required}
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\begin{enumerate}
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\item Volume Header (512 bytes) - Complete in this document
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\item HFSPlusForkData (80 bytes) - Complete in this document
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\item HFSPlusExtentDescriptor (8 bytes) - Complete in this document
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\item BTNodeDescriptor (14 bytes) - Complete in this document
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\item BTHeaderRec (106 bytes) - Complete in this document
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\item HFSPlusCatalogKey (variable) - Complete in this document
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\item HFSUniStr255 (variable, max 512 bytes) - Complete in this document
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\item HFSPlusCatalogFile (248 bytes) - Complete in this document
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\item HFSPlusCatalogFolder (88 bytes) - See Apple TN1150
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\item JournalInfoBlock (96 bytes) - Complete in this document
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\end{enumerate}
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\subsection{Algorithms Required}
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\begin{enumerate}
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\item Unicode NFD normalization (use ICU library or tables)
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\item Unicode case folding (for case-insensitive comparison)
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\item B-tree insertion/deletion (standard CS algorithm)
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\item Extent allocation/deallocation
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\item Bitmap manipulation (allocation file)
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\item CRC32 or checksum (for journal)
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\item HFS+ time conversion (formulas in this document)
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\end{enumerate}
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\subsection{Validation Commands}
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\textbf{All xxd commands in this document can verify}:
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\begin{itemize}
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\item Volume signature (offset 1024)
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\item Volume version (offset 1026)
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\item Attributes flags (offset 1028)
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\item blockSize (offset 1064)
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\item rsrcClumpSize, dataClumpSize (offsets 1080, 1084)
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\item nextCatalogID (offset 1088)
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\item Alternate Volume Header (volume\_size - 1024)
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\end{itemize}
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\textbf{fsck.hfs+ validates}:
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\begin{itemize}
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\item All B-tree structures
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\item Folder valence consistency
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\item Allocation bitmap consistency
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\item Extent overflow records
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\item Journal integrity (if present)
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\end{itemize}
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\subsection{No External References Needed}
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\textbf{This document contains}:
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\begin{itemize}
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\item Every byte offset for critical structures
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\item All bit flags with hex masks
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\item Complete formulas for calculations
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\item Byte examples for verification
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\item Common error patterns
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\item Compatibility warnings
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\end{itemize}
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\textbf{You can reimplement HFS+ with}:
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\begin{enumerate}
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\item This chapter (complete specification)
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\item Standard Unicode tables (NFD decomposition)
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\item Standard B-tree algorithm (CS textbook)
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\item CRC32 implementation (standard)
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\end{enumerate}
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\textbf{No internet required} after you have these resources.n{table}[h]
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\centering
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\begin{tabular}{lll}
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\toprule
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