Simple implementation of levenshtein distance for MySQL

Makefile

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+levenshtein.so: levenshtein.c
+	gcc -Wall -fpic $(CFLAGS) -I/usr/include/mysql -shared -o $@ $<
+
+install: levenshtein.so
+	cp levenshtein.so /usr/lib/mysql/plugin/
+	echo 'CREATE FUNCTION levenshtein RETURNS INTEGER soname "levenshtein.so"' | mysql -u root -p
+
+clean:
+	rm -f levenshtein.so

levenshtein.c

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+/*
+#############################################################################
+# License: GNU GPL 2.0 or later
+#############################################################################
+# Header from the template:
+#
+# http://dev.mysql.com/sources/doxygen/mysql-5.1/udf__example_8c-source.html
+#############################################################################
+*/
+
+#include <my_global.h>
+#include <my_sys.h>
+#include <m_string.h>
+#include <mysql.h>
+#include <ctype.h>
+#include <stdio.h>
+
+#ifdef HAVE_DLOPEN
+my_bool levenshtein_init(UDF_INIT *initid, UDF_ARGS *args, char *message);
+longlong levenshtein(UDF_INIT *initid, UDF_ARGS *args, char *is_null,
+                    char *error);
+#endif
+longlong levenshtein(UDF_INIT *initid __attribute__((unused)), UDF_ARGS *args,
+                    char *is_null __attribute__((unused)),
+                    char *error __attribute__((unused)))
+{
+  if (args->arg_count != 2)
+    return -3;
+  if (args->arg_type[0] !=  STRING_RESULT)
+    return -1;
+  if (args->arg_type[1] !=  STRING_RESULT)
+    return -2;
+  const char *wA = args->args[0];
+  const char *wB = args->args[1];
+  int m = args->lengths[0];
+  int n = args->lengths[1];
+  int cost =0;
+  int i;
+  int j;
+  longlong D[(m+1)][(n+1)];
+  for (i = 0; i <= m; i++){
+    D[i][0] = i;
+  }
+  for (i = 1; i <= n; i++){
+    D[0][i] = i;
+  }
+  for (i = 1; i <= m; i++){
+    for (j = 1; j <= n; j++){
+      if (wA[i-1] == wB[j-1])
+        cost = 0;
+      else
+        cost = 1;
+      if ((D[i-1][j] + 1 <= D[i][j-1] + 1) && (D[i-1][j] + 1 <= D[i-1][j-1] + cost)){ // min(D[i-1][j]+1)
+        D[i][j] = D[i-1][j] + 1;
+      }
+      else{
+        if ((D[i][j-1] + 1 <= D[i-1][j] + 1) && (D[i][j-1] + 1 <= D[i-1][j-1] + cost))  { // min(D[i][j-1]+1
+          D[i][j] = D[i][j-1] + 1;
+        }
+        else{
+           D[i][j] = D[i-1][j-1] + cost;                                                // min(D[i-1][j-1]+1
+        }
+      }
+    }
+  }
+  return D[m][n];
+}
+my_bool levenshtein_init(UDF_INIT *initid __attribute__((unused)),
+                        UDF_ARGS *args __attribute__((unused)),
+                        char *message __attribute__((unused)))
+{
+  return 0;
+}