79 lines
1.9 KiB
C
79 lines
1.9 KiB
C
char index_adjust[8] = { -1, -1, -1, -1, 2, 4, 6, 8 };
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short step_size[89] = {
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7, 8, 9, 10, 11, 12, 13, 14, 16, 17,
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19, 21, 23, 25, 28, 31, 34, 37, 41, 45,
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50, 55, 60, 66, 73, 80, 88, 97, 107, 118,
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130, 143, 157, 173, 190, 209, 230, 253, 279, 307,
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337, 371, 408, 449, 494, 544, 598, 658, 724, 796,
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876, 963, 1060, 1166, 1282, 1411, 1552, 1707, 1878, 2066,
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2272, 2499, 2749, 3024, 3327, 3660, 4026, 4428, 4871, 5358,
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5894, 6484, 7132, 7845, 8630, 9493, 10442, 11487, 12635, 13899,
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15289, 16818, 18500, 20350, 22385, 24623, 27086, 29794, 32767
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};
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int pred_val = 0;
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int step_idx = 0;
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/* This code is "borrowed" from the ALSA library
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http://www.alsa-project.org */
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short adpcm_decode_sample(char code) {
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short pred_diff; /* Predicted difference to next sample */
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short step; /* holds previous step_size value */
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char sign;
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int i;
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/* Separate sign and magnitude */
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sign = code & 0x8;
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code &= 0x7;
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/*
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* Computes pred_diff = (code + 0.5) * step / 4,
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* but see comment in adpcm_coder.
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*/
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step = step_size[step_idx];
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/* Compute difference and new predicted value */
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pred_diff = step >> 3;
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for (i = 0x4; i; i >>= 1, step >>= 1) {
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if (code & i) {
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pred_diff += step;
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}
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}
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pred_val += (sign) ? -pred_diff : pred_diff;
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/* Clamp output value */
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if (pred_val > 32767) {
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pred_val = 32767;
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} else if (pred_val < -32768) {
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pred_val = -32768;
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}
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/* Find new step_size index value */
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step_idx += index_adjust[(int) code];
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if (step_idx < 0) {
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step_idx = 0;
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} else if (step_idx > 88) {
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step_idx = 88;
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}
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return pred_val;
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}
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void adpcm_init() {
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pred_val = 0;
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step_idx = 0;
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}
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void adpcm_decode(unsigned char* input, unsigned int input_size, short** output) {
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unsigned int i;
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for (i = 0; i < input_size; ++i) {
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unsigned char two_samples = input[i];
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*((*output)++) = adpcm_decode_sample(two_samples >> 4);
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*((*output)++) = adpcm_decode_sample(two_samples & 15);
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}
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}
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