From 572e8b52e16ddc9fa70f46b19168c391d803cf98 Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Fri, 26 Jul 2019 17:20:32 -0400
Subject: [PATCH] At the cost of extra storage, attempts to simplify away
 potential rounding issues around the index hole.

---
 Storage/Disk/Drive.cpp | 11 ++++++++---
 Storage/Disk/Drive.hpp |  4 ++++
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/Storage/Disk/Drive.cpp b/Storage/Disk/Drive.cpp
index e2844c2a4..4142884d6 100644
--- a/Storage/Disk/Drive.cpp
+++ b/Storage/Disk/Drive.cpp
@@ -38,10 +38,15 @@ Drive::Drive(int input_clock_rate, int revolutions_per_minute, int number_of_hea
 Drive::Drive(int input_clock_rate, int number_of_heads) : Drive(input_clock_rate, 300, number_of_heads) {}
 
 void Drive::set_rotation_speed(float revolutions_per_minute) {
-	const float new_rotational_multiplier = 60.0f / revolutions_per_minute;
+	// Rationalise the supplied speed so that cycles_per_revolution_ is exact.
+	cycles_per_revolution_ = int(0.5f + float(get_input_clock_rate()) * 60.0f / revolutions_per_minute);
+
+	// From there derive the appropriate rotational multiplier and possibly update the
+	// count of cycles since the index hole proportionally.
+	const float new_rotational_multiplier = float(cycles_per_revolution_) / float(get_input_clock_rate());
 	cycles_since_index_hole_ *= new_rotational_multiplier / rotational_multiplier_;
 	rotational_multiplier_ = new_rotational_multiplier;
-	cycles_since_index_hole_ %= int(get_input_clock_rate() * rotational_multiplier_);
+	cycles_since_index_hole_ %= cycles_per_revolution_;
 }
 
 Drive::~Drive() {
@@ -358,7 +363,7 @@ void Drive::end_writing() {
 			}
 		}
 		patched_track_->add_segment(write_start_time_, write_segment_, clamp_writing_to_index_hole_);
-		cycles_since_index_hole_ %= int(get_input_clock_rate() * rotational_multiplier_);
+		cycles_since_index_hole_ %= cycles_per_revolution_;
 		invalidate_track();
 	}
 }
diff --git a/Storage/Disk/Drive.hpp b/Storage/Disk/Drive.hpp
index 183976938..5bf2c0bde 100644
--- a/Storage/Disk/Drive.hpp
+++ b/Storage/Disk/Drive.hpp
@@ -198,6 +198,10 @@ class Drive: public ClockingHint::Source, public TimedEventLoop {
 		// a new track.
 		int cycles_since_index_hole_ = 0;
 
+		// The number of cycles that should fall within one revolution at the
+		// current rotation speed.
+		int cycles_per_revolution_ = 1;
+
 		// A record of head position and active head.
 		HeadPosition head_position_;
 		int head_ = 0;