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CLK/Storage/Disk/Track/PCMSegment.cpp
2017-09-22 22:39:23 -04:00

114 lines
4.1 KiB
C++

//
// PCMSegment.cpp
// Clock Signal
//
// Created by Thomas Harte on 17/12/2016.
// Copyright © 2016 Thomas Harte. All rights reserved.
//
#include "PCMSegment.hpp"
using namespace Storage::Disk;
PCMSegmentEventSource::PCMSegmentEventSource(const PCMSegment &segment) :
segment_(new PCMSegment(segment)) {
// add an extra bit of storage at the bottom if one is going to be needed;
// events returned are going to be in integral multiples of the length of a bit
// other than the very first and very last which will include a half bit length
if(segment_->length_of_a_bit.length&1) {
segment_->length_of_a_bit.length <<= 1;
segment_->length_of_a_bit.clock_rate <<= 1;
}
// load up the clock rate once only
next_event_.length.clock_rate = segment_->length_of_a_bit.clock_rate;
// set initial conditions
reset();
}
PCMSegmentEventSource::PCMSegmentEventSource(const PCMSegmentEventSource &original) {
// share underlying data with the original
segment_ = original.segment_;
// load up the clock rate and set initial conditions
next_event_.length.clock_rate = segment_->length_of_a_bit.clock_rate;
reset();
}
void PCMSegmentEventSource::reset() {
// start with the first bit to be considered the zeroth, and assume that it'll be
// flux transitions for the foreseeable
bit_pointer_ = 0;
next_event_.type = Track::Event::FluxTransition;
}
Storage::Disk::Track::Event PCMSegmentEventSource::get_next_event() {
// track the initial bit pointer for potentially considering whether this was an
// initial index hole or a subsequent one later on
size_t initial_bit_pointer = bit_pointer_;
// if starting from the beginning, pull half a bit backward, as if the initial bit
// is set, it should be in the centre of its window
next_event_.length.length = bit_pointer_ ? 0 : -(segment_->length_of_a_bit.length >> 1);
// search for the next bit that is set, if any
const uint8_t *segment_data = segment_->data.data();
while(bit_pointer_ < segment_->number_of_bits) {
int bit = segment_data[bit_pointer_ >> 3] & (0x80 >> (bit_pointer_&7));
bit_pointer_++; // so this always points one beyond the most recent bit returned
next_event_.length.length += segment_->length_of_a_bit.length;
// if this bit is set, return the event
if(bit) return next_event_;
}
// if the end is reached without a bit being set, it'll be index holes from now on
next_event_.type = Track::Event::IndexHole;
// test whether this is the very first time that bits have been exhausted. If so then
// allow an extra half bit's length to run from the position of the potential final transition
// event to the end of the segment. Otherwise don't allow any extra time, as it's already
// been consumed
if(initial_bit_pointer <= segment_->number_of_bits) {
next_event_.length.length += (segment_->length_of_a_bit.length >> 1);
bit_pointer_++;
}
return next_event_;
}
Storage::Time PCMSegmentEventSource::get_length() {
return segment_->length_of_a_bit * segment_->number_of_bits;
}
Storage::Time PCMSegmentEventSource::seek_to(const Time &time_from_start) {
// test for requested time being beyond the end
Time length = get_length();
if(time_from_start >= length) {
next_event_.type = Track::Event::IndexHole;
bit_pointer_ = segment_->number_of_bits+1;
return length;
}
// if not beyond the end then make an initial assumption that the next thing encountered will be a flux transition
next_event_.type = Track::Event::FluxTransition;
// test for requested time being before the first bit
Time half_bit_length = segment_->length_of_a_bit;
half_bit_length.length >>= 1;
if(time_from_start < half_bit_length) {
bit_pointer_ = 0;
Storage::Time zero;
return zero;
}
// adjust for time to get to bit zero and determine number of bits in;
// bit_pointer_ always records _the next bit_ that might trigger an event,
// so should be one beyond the one reached by a seek.
Time relative_time = time_from_start - half_bit_length;
bit_pointer_ = 1 + (relative_time / segment_->length_of_a_bit).get_unsigned_int();
// map up to the correct amount of time
return half_bit_length + segment_->length_of_a_bit * (unsigned int)(bit_pointer_ - 1);
}