X-Git-Url: https://git.sesse.net/?a=blobdiff_plain;f=hamming.c;h=33376f31993bd4b1ed25cd45355984b7ed1bcb4f;hb=069b121a26d0b1ca1ad43a747d441b5882cdb405;hp=03714582b4ac5018e95551d86798f4d8dbf6fc50;hpb=c9fad713cb2e032540dcee980cb7241b6123205d;p=hamming

diff --git a/hamming.c b/hamming.c
index 0371458..33376f3 100644
--- a/hamming.c
+++ b/hamming.c
@@ -2,6 +2,7 @@
 
 #define DATA_BITS    11
 #define PARITY_BITS  5
+#define EXTRA_BIT_POSITION (PARITY_BITS - 1)
 #define CODE_BITS    (DATA_BITS + PARITY_BITS)
 #define NUM_DATA_WORDS (1 << DATA_BITS)
 
@@ -9,7 +10,8 @@ unsigned char hamming_lookup[NUM_DATA_WORDS];
 
 /* 
  * Needed since we store all the parity at the end of the word, not at the expected
- * power-of-two bit positions.
+ * power-of-two bit positions. This is the inverse of the mapping
+ * (0..15) -> (0, 8, 4, 2, 1, the rest in ascending order)
  */
 unsigned char permutation_table[CODE_BITS] = {
 	0, 4, 3, 5, 2, 6, 7, 8, 1, 9, 10, 11, 12, 13, 14, 15
@@ -21,20 +23,20 @@ unsigned generate_parity(unsigned data)
 	unsigned parity[PARITY_BITS];
 	unsigned i;
 
-	parity[4] = 0;
+	parity[EXTRA_BIT_POSITION] = 0;
 	
 	for (i = 0; i < DATA_BITS; ++i) {
 		bits[i] = (data & (1 << i)) ? 1 : 0;
-		parity[4] ^= bits[i];
+		parity[EXTRA_BIT_POSITION] ^= bits[i];
 	}
 
 	parity[0] = bits[0] ^ bits[1] ^ bits[3] ^ bits[4] ^ bits[6] ^ bits[8] ^ bits[10];
 	parity[1] = bits[0] ^ bits[2] ^ bits[3] ^ bits[5] ^ bits[6] ^ bits[9] ^ bits[10];
 	parity[2] = bits[1] ^ bits[2] ^ bits[3] ^ bits[7] ^ bits[8] ^ bits[9] ^ bits[10];
 	parity[3] = bits[4] ^ bits[5] ^ bits[6] ^ bits[7] ^ bits[8] ^ bits[9] ^ bits[10];
-	parity[4] ^= parity[0] ^ parity[1] ^ parity[2] ^ parity[3];
+	parity[EXTRA_BIT_POSITION] ^= parity[0] ^ parity[1] ^ parity[2] ^ parity[3];
 
-	return parity[4] | (parity[3] << 1) | (parity[2] << 2) | (parity[1] << 3) | (parity[0] << 4);
+	return parity[EXTRA_BIT_POSITION] | (parity[3] << 1) | (parity[2] << 2) | (parity[1] << 3) | (parity[0] << 4);
 }
 
 unsigned make_codeword(unsigned data)
@@ -102,13 +104,13 @@ unsigned correct_single_bit_error(unsigned code)
 	unsigned parity[PARITY_BITS];
 	unsigned i, bp = 0;
 
-	parity[4] = 0;
+	parity[EXTRA_BIT_POSITION] = 0;
 
 	for (i = 0; i < CODE_BITS; ++i) {
 		bits[i] = (code & (1 << i)) ? 1 : 0;
 	}
 	for (i = 1; i < CODE_BITS; ++i) {
-		parity[4] ^= bits[i];
+		parity[EXTRA_BIT_POSITION] ^= bits[i];
 	}
 
 	parity[0] = bits[PARITY_BITS+0] ^ bits[PARITY_BITS+1] ^ bits[PARITY_BITS+3] ^ bits[PARITY_BITS+4] ^ bits[PARITY_BITS+6] ^ bits[PARITY_BITS+8] ^ bits[PARITY_BITS+10];
@@ -125,11 +127,11 @@ unsigned correct_single_bit_error(unsigned code)
 	if (bp != 0) {
 		/* flip the wrong bit */
 		code ^= (1 << permutation_table[bp]);
-		parity[4] ^= 1;
+		parity[EXTRA_BIT_POSITION] ^= 1;
 	}
 
 	/* recompute the lower parity */
-	return (code & ~1) | parity[4];
+	return (code & ~1) | parity[EXTRA_BIT_POSITION];
 }
 
 void check_zero_bit_detection()