62 lines
2.6 KiB
C
62 lines
2.6 KiB
C
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/* Since the Norpak bundle FEC uses the same FEC for vertical and
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horizontal lines, we need to do several of the same operations
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on vertical and horizontal lines. However, it's difficult
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to share the code, because operations which work on horizontal
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lines want to assume that the bytes in the line are consecutive,
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and operations which work on vertical lines want to assume that
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the bytes in the line are separated by sizeof(Packet).
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We could just pass in a "stride" (1 for horizontal, sizeof(Packet)
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for vertical); but this would involve a multiplication in both
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the horizontal and vertical inner loops. Instead, we write the
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code once, and rely on various preprocessor shenanigans to
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create two different versions; in the horizontal version, any
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decent compiler will optimize away the multiplications by 1.
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(Basically, we just #include this file into nabtslib.c twice
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with different preprocessor #define's in effect; once for the
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horizontal and once for the vertical operations.) */
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/* STRIDE is #define'd in nabtslib.c to be either 1 or sizeof(Packet) */
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#define VALS(x) (vals[STRIDE*(x)])
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/* This code generates compute_csum_vert and compute_csum_horiz.
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Given a (horizontal or vertical) line (actually all but the
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last two bytes of the line), it computes the correct checksum
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for the line (the correct last two bytes). */
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int STRIDENAM(compute_csum)(unsigned char *vals, int len) {
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int check = 0;
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int i;
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for (i = 0; i < len; i++) {
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/* If VALS(i) is zero, then the byte has no effect on the checksum. */
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if (VALS(i)) {
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int bcheck = galois_log[VALS(i)];
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int offs0 = log_norpak_coeffs[0][i];
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int offs1 = log_norpak_coeffs[1][i];
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check ^= galois_exp[bcheck+offs0]<<8 | galois_exp[bcheck+offs1];
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}
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}
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return check;
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}
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/* This code generates check_checksum_vert and check_checksum_horiz.
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Given a (horizontal or vertical) line, it computes the error for
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the line (the XOR of the given checksum and the computed checksum).
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It then calls update_fec_inf() to determine whether the line is OK
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(no error), whether the error can be explained as a single-byte error,
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or whether the error must be at least two-byte. */
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fec_stat STRIDENAM(check_checksum)(unsigned char *vals, int len, fec_info *inf) {
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int check = STRIDENAM(compute_csum)(vals, len);
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inf->err = check ^ (VALS(len)<<8 | VALS(len+1));
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inf->errl[0] = galois_log[inf->err>>8];
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inf->errl[1] = galois_log[inf->err&0xff];
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/* The following line makes inf->status no longer be fec_status_missing
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(update_fec_inf does nothing for missing lines) */
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inf->status = fec_status_ok;
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return update_fec_inf(inf, len);
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}
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