From 3a4c2e46c10cddaad35f690e27a5f58c78f41902 Mon Sep 17 00:00:00 2001
From: Chris Pressey <cpressey@catseye.tc>
Date: Tue, 11 Dec 2018 17:35:01 +0000
Subject: [PATCH] `inc` and `dec` on a known range usually keeps it known, now.

---
 HISTORY.md                    |  2 ++
 eg/rudiments/vector-table.60p |  8 -------
 src/sixtypical/analyzer.py    | 15 +++++++++++-
 tests/SixtyPical Analysis.md  | 43 +++++++++++++++++++++++++++++++++++
 4 files changed, 59 insertions(+), 9 deletions(-)

diff --git a/HISTORY.md b/HISTORY.md
index c15b971..dda7e4c 100644
--- a/HISTORY.md
+++ b/HISTORY.md
@@ -7,6 +7,8 @@ History of SixtyPical
 *   Syntactically, `goto` may only appear at the end of a block.
     It need no longer be the final instruction in a routine,
     as long as the type context is consistent at every exit.
+*   When the range of a location is known, `inc` and `dec`
+    on it will usually shift the known instead of invalidating it.
 *   `cmp` instruction can now perform a 16-bit unsigned comparison
     of `word` memory locations (at the cost of trashing `a`.)
 *   Fixed pathological memory use in the lexical scanner - should
diff --git a/eg/rudiments/vector-table.60p b/eg/rudiments/vector-table.60p
index b6cce2a..6d6e98a 100644
--- a/eg/rudiments/vector-table.60p
+++ b/eg/rudiments/vector-table.60p
@@ -3,14 +3,6 @@
 // Prints "AABAB".
 //
 
-// TODO: this doesn't pass the analyzer currently, which suggests a bug.
-//
-//     RangeExceededError: Possible range of x:byte (0, 255) exceeds
-//     acceptable range of vectors:vector table[32] (0, 31) (in main, line 57)
-//
-// (despite various attempts to work around by calling a setup routine, etc.)
-// It should really be able to figure out that the range of x is 0..4 there.
-
 vector routine
   trashes a, z, n
     print
diff --git a/src/sixtypical/analyzer.py b/src/sixtypical/analyzer.py
index cabff6d..9837424 100644
--- a/src/sixtypical/analyzer.py
+++ b/src/sixtypical/analyzer.py
@@ -597,7 +597,20 @@ class Analyzer(object):
             else:
                 self.assert_type(TYPE_BYTE, dest)
                 context.set_written(dest, FLAG_Z, FLAG_N)
-                context.invalidate_range(dest)
+                bottom = context.get_bottom_of_range(dest)
+                top = context.get_top_of_range(dest)
+                if opcode == 'inc':
+                    if bottom == top and top < 255:
+                        context.set_range(dest, bottom + 1, top + 1)
+                    else:
+                        context.invalidate_range(dest)
+                elif opcode == 'dec':
+                    if bottom == top and bottom > 0:
+                        context.set_range(dest, bottom - 1, top - 1)
+                    else:
+                        context.invalidate_range(dest)
+                else:
+                    raise NotImplementedError
         elif opcode in ('shl', 'shr'):
             context.assert_meaningful(dest, FLAG_C)
             if isinstance(dest, IndexedRef):
diff --git a/tests/SixtyPical Analysis.md b/tests/SixtyPical Analysis.md
index 071553b..0331e7e 100644
--- a/tests/SixtyPical Analysis.md	
+++ b/tests/SixtyPical Analysis.md	
@@ -762,6 +762,49 @@ no longer be guaranteed.
     | }
     ? RangeExceededError
 
+When the range of a location is known, incrementing or
+decrementing that location's value will shift the known
+range.  It will not invalidate it unless the known range
+is at the limits of the possible ranges for the type.
+
+    | vector routine
+    |   trashes a, z, n
+    |     print
+    | 
+    | vector (routine
+    |   trashes a, z, n)
+    |     table[32] vectors
+    | 
+    | define main routine
+    |   inputs vectors, print
+    |   outputs vectors
+    |   trashes print, a, x, z, n, c
+    | {
+    |     ld x, 0
+    |     inc x
+    |     copy print, vectors + x
+    | }
+    = ok
+
+    | vector routine
+    |   trashes a, z, n
+    |     print
+    | 
+    | vector (routine
+    |   trashes a, z, n)
+    |     table[32] vectors
+    | 
+    | define main routine
+    |   inputs vectors, print
+    |   outputs vectors
+    |   trashes print, a, x, z, n, c
+    | {
+    |     ld x, 32
+    |     dec x
+    |     copy print, vectors + x
+    | }
+    = ok
+
 ### add ###
 
 Can't `add` from or to a memory location that isn't initialized.