+static void create_or_reset_lcs_table()
+{
+ int i;
+
+ if (L != NULL) {
+ memset(*L, 0, sizeof(int) * MAX_SSDIFF_SIZE);
+ return;
+ }
+
+ // xcalloc will die if we ran out of memory;
+ // not very helpful for debugging
+ L = (int**)xcalloc(MAX_SSDIFF_M, sizeof(int *));
+ *L = (int*)xcalloc(MAX_SSDIFF_SIZE, sizeof(int));
+
+ for (i = 1; i < MAX_SSDIFF_M; i++) {
+ L[i] = *L + i * MAX_SSDIFF_N;
+ }
+}
+
+static char *longest_common_subsequence(char *A, char *B)
+{
+ int i, j, ri;
+ int m = strlen(A);
+ int n = strlen(B);
+ int tmp1, tmp2;
+ int lcs_length;
+ char *result;
+
+ // We bail if the lines are too long
+ if (m >= MAX_SSDIFF_M || n >= MAX_SSDIFF_N)
+ return NULL;
+
+ create_or_reset_lcs_table();
+
+ for (i = m; i >= 0; i--) {
+ for (j = n; j >= 0; j--) {
+ if (A[i] == '\0' || B[j] == '\0') {
+ L[i][j] = 0;
+ } else if (A[i] == B[j]) {
+ L[i][j] = 1 + L[i + 1][j + 1];
+ } else {
+ tmp1 = L[i + 1][j];
+ tmp2 = L[i][j + 1];
+ L[i][j] = (tmp1 > tmp2 ? tmp1 : tmp2);
+ }
+ }
+ }
+
+ lcs_length = L[0][0];
+ result = xmalloc(lcs_length + 2);
+ memset(result, 0, sizeof(*result) * (lcs_length + 2));
+
+ ri = 0;
+ i = 0;
+ j = 0;
+ while (i < m && j < n) {
+ if (A[i] == B[j]) {
+ result[ri] = A[i];
+ ri += 1;
+ i += 1;
+ j += 1;
+ } else if (L[i + 1][j] >= L[i][j + 1]) {
+ i += 1;
+ } else {
+ j += 1;
+ }
+ }
+
+ return result;
+}
+