find common prefix and suffix of two strings

kittens/diff/speedup.c

@@ -0,0 +1,52 @@
+/*
+ * speedup.c
+ * Copyright (C) 2018 Kovid Goyal <kovid at kovidgoyal.net>
+ *
+ * Distributed under terms of the GPL3 license.
+ */
+
+#include "data-types.h"
+
+static PyObject*
+changed_center(PyObject *self UNUSED, PyObject *args) {
+    unsigned int prefix_count = 0, suffix_count = 0;
+    PyObject *lp, *rp;
+    if (!PyArg_ParseTuple(args, "UU", &lp, &rp)) return NULL;
+    const size_t left_len = PyUnicode_GET_LENGTH(lp), right_len = PyUnicode_GET_LENGTH(rp);
+
+#define R(which, index) PyUnicode_READ(PyUnicode_KIND(which), PyUnicode_DATA(which), index)
+    while(prefix_count < MIN(left_len, right_len)) {
+        if (R(lp, prefix_count) != R(rp, prefix_count)) break;
+        prefix_count++;
+    }
+    if (left_len && right_len && prefix_count < MIN(left_len, right_len)) {
+        while(suffix_count < MIN(left_len - prefix_count, right_len - prefix_count)) {
+            if(R(lp, left_len - 1 - suffix_count) != R(rp, right_len - 1 - suffix_count)) break;
+            suffix_count++;
+        }
+    }
+#undef R
+    return Py_BuildValue("II", prefix_count, suffix_count);
+}
+
+static PyMethodDef module_methods[] = {
+    {"changed_center", (PyCFunction)changed_center, METH_VARARGS, ""},
+    {NULL, NULL, 0, NULL}        /* Sentinel */
+};
+
+static struct PyModuleDef module = {
+   .m_base = PyModuleDef_HEAD_INIT,
+   .m_name = "diff_speedup",   /* name of module */
+   .m_doc = NULL,
+   .m_size = -1,
+   .m_methods = module_methods
+};
+
+EXPORTED PyMODINIT_FUNC
+PyInit_diff_speedup(void) {
+    PyObject *m;
+
+    m = PyModule_Create(&module);
+    if (m == NULL) return NULL;
+    return m;
+}