From a3e00249805fa4758d9abe101119c7f33607b913 Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Fri, 21 Jul 2017 18:55:03 -0400
Subject: [PATCH] Chopped time accumulation out of the default `Tape` process
 because it's proving to be sufficiently expensive for a TZX as not to be
 worthwhile. Introduced a cheaper position capturing/restoring method.

---
 Machines/ZX8081/ZX8081.cpp |  4 ++--
 Storage/Tape/Tape.cpp      | 39 +++++++++++++++++++++++++++++---------
 Storage/Tape/Tape.hpp      | 25 ++++++++++++++++++++----
 3 files changed, 53 insertions(+), 15 deletions(-)

diff --git a/Machines/ZX8081/ZX8081.cpp b/Machines/ZX8081/ZX8081.cpp
index 83f6b8e54..23fe0db44 100644
--- a/Machines/ZX8081/ZX8081.cpp
+++ b/Machines/ZX8081/ZX8081.cpp
@@ -130,7 +130,7 @@ int Machine::perform_machine_cycle(const CPU::Z80::PartialMachineCycle &cycle) {
 		case CPU::Z80::PartialMachineCycle::ReadOpcodeWait:
 			// Check for use of the fast tape hack.
 			if(use_fast_tape_hack_ && address == tape_trap_address_ && tape_player_.has_tape()) {
-				Storage::Time time = tape_player_.get_tape()->get_current_time();
+				uint64_t prior_offset = tape_player_.get_tape()->get_offset();
 				int next_byte = parser_.get_next_byte(tape_player_.get_tape());
 				if(next_byte != -1) {
 					uint16_t hl = get_value_of_register(CPU::Z80::Register::HL);
@@ -144,7 +144,7 @@ int Machine::perform_machine_cycle(const CPU::Z80::PartialMachineCycle &cycle) {
 					tape_advance_delay_ = 1000;
 					return 0;
 				} else {
-					tape_player_.get_tape()->seek(time);
+					tape_player_.get_tape()->set_offset(prior_offset);
 				}
 			}
 
diff --git a/Storage/Tape/Tape.cpp b/Storage/Tape/Tape.cpp
index 24528e58b..de4892b35 100644
--- a/Storage/Tape/Tape.cpp
+++ b/Storage/Tape/Tape.cpp
@@ -20,22 +20,43 @@ TapePlayer::TapePlayer(unsigned int input_clock_rate) :
 #pragma mark - Seeking
 
 void Storage::Tape::Tape::seek(Time &seek_time) {
-	current_time_.set_zero();
-	next_time_.set_zero();
-	while(next_time_ <= seek_time) get_next_pulse();
+	Time next_time(0);
+	reset();
+	while(next_time <= seek_time) {
+		get_next_pulse();
+		next_time += pulse_.length;
+	}
+}
+
+Storage::Time Tape::get_current_time() {
+	Time time(0);
+	uint64_t steps = get_offset();
+	reset();
+	while(steps--) {
+		get_next_pulse();
+		time += pulse_.length;
+	}
+	return time;
 }
 
 void Storage::Tape::Tape::reset() {
-	current_time_.set_zero();
-	next_time_.set_zero();
+	offset_ = 0;
 	virtual_reset();
 }
 
 Tape::Pulse Tape::get_next_pulse() {
-	Tape::Pulse pulse = virtual_get_next_pulse();
-	current_time_ = next_time_;
-	next_time_ += pulse.length;
-	return pulse;
+	pulse_ = virtual_get_next_pulse();
+	offset_++;
+	return pulse_;
+}
+
+uint64_t Tape::get_offset() {
+	return offset_;
+}
+
+void Tape::set_offset(uint64_t offset) {
+	reset();
+	while(offset--) get_next_pulse();
 }
 
 #pragma mark - Player
diff --git a/Storage/Tape/Tape.hpp b/Storage/Tape/Tape.hpp
index 2cd6c028b..6f72ee4f7 100644
--- a/Storage/Tape/Tape.hpp
+++ b/Storage/Tape/Tape.hpp
@@ -52,16 +52,33 @@ class Tape {
 		/// @returns @c true if the tape has progressed beyond all recorded content; @c false otherwise.
 		virtual bool is_at_end() = 0;
 
-		/// @returns the amount of time preceeding the most recently-returned pulse.
-		virtual Time get_current_time() { return current_time_; }
+		/*!
+			Returns a numerical representation of progression into the tape. Precision is arbitrary but
+			required to be at least to the whole pulse. Greater numbers are later than earlier numbers,
+			but not necessarily continuous.
+		*/
+		virtual uint64_t get_offset();
 
-		/// Advances or reverses the tape to the last time before or at @c time from which a pulse starts.
+		/*!
+			Moves the tape to the first time at which the specified offset would be returned by get_offset.
+		*/
+		virtual void set_offset(uint64_t);
+
+		/*!
+			Calculates and returns the amount of time that has elapsed since the time began. Potentially expensive.
+		*/
+		virtual Time get_current_time();
+
+		/*!
+			Seeks to @c time. Potentially expensive.
+		*/
 		virtual void seek(Time &time);
 
 		virtual ~Tape() {};
 
 	private:
-		Time current_time_, next_time_;
+		uint64_t offset_;
+		Tape::Pulse pulse_;
 
 		virtual Pulse virtual_get_next_pulse() = 0;
 		virtual void virtual_reset() = 0;