Sumner Evans
83 lines
2.7 KiB
Go
83 lines
2.7 KiB
Go
// mautrix-telegram - A Matrix-Telegram puppeting bridge.
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// Copyright (C) 2025 Sumner Evans
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//
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// This program is free software: you can redistribute it and/or modify
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// it under the terms of the GNU Affero General Public License as published by
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// the Free Software Foundation, either version 3 of the License, or
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// (at your option) any later version.
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//
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// This program is distributed in the hope that it will be useful,
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// but WITHOUT ANY WARRANTY; without even the implied warranty of
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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// GNU Affero General Public License for more details.
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//
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// You should have received a copy of the GNU Affero General Public License
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// along with this program. If not, see <https://www.gnu.org/licenses/>.
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// Package waveform implements encoding and decoding of a Telegram waveform.
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//
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// Telegram waveforms consist of packed 5-bit values. The values are packed
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// into a byte stream, meaning that the actual values cross the byte boundary.
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//
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// The following diagram explains the format:
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//
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// [210|43210][0|43210|43][3210|4321][10|43210|4]...
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// [111|00000][3|22222|11][4444|3333][66|55555|4]...
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//
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// Explanation of diagram:
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// - The []'s enclose byte boundaries.
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// - The |s represent separation between waveform values.
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// - The numbers in the first row indicate the binary power.
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// - The numbers in the second row indicate the corresponding waveform index.
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package waveform
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import "math"
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// NormalizeWaveform normalizes a waveform by bounding the values to the range
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// [0, 32] which is required for the encoding to work.
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func NormalizeWaveform(waveform []int) (normalized []byte) {
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normalized = make([]byte, len(waveform))
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var waveformMax int
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for _, v := range waveform {
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waveformMax = max(waveformMax, v)
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}
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for i, v := range waveform {
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normalized[i] = byte(math.Round(float64(v) / float64(max(waveformMax/256, 1))))
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}
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return
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}
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// Encode normalizes and encodes the input Matrix waveform into a Telegram
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// waveform.
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func Encode(waveform []int) []byte {
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bytesCount := (len(waveform)*5 + 7) / 8
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result := make([]byte, bytesCount+1)
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var bitShift int
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for i, v := range NormalizeWaveform(waveform) {
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result[i*5/8] |= v << bitShift
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result[i*5/8+1] |= v >> (8 - bitShift)
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bitShift = (bitShift + 5) % 8
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}
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return result[:bytesCount]
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}
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// Decode decodes a Telegram waveform into a waveform usable by Matrix.
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func Decode(waveform []byte) []int {
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numValues := len(waveform) * 8 / 5
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result := make([]int, numValues)
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var bitShift int
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for i := 0; i < numValues; i++ {
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var val byte
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val |= waveform[i*5/8] >> bitShift
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if i*5/8+1 < len(waveform) {
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val |= waveform[i*5/8+1] << (8 - bitShift)
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}
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result[i] = int(val) & 0b00011111
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bitShift = (bitShift + 5) % 8
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}
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return result
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}
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