Refactor ALL the things!

src/parsers/difference.c

@@ -0,0 +1,47 @@
+#include "parser_internal.h"
+
+typedef struct {
+  const HParser *p1;
+  const HParser *p2;
+} HTwoParsers;
+
+static HParseResult* parse_difference(void *env, HParseState *state) {
+  HTwoParsers *parsers = (HTwoParsers*)env;
+  // cache the initial state of the input stream
+  HInputStream start_state = state->input_stream;
+  HParseResult *r1 = h_do_parse(parsers->p1, state);
+  // if p1 failed, bail out early
+  if (NULL == r1) {
+    return NULL;
+  } 
+  // cache the state after parse #1, since we might have to back up to it
+  HInputStream after_p1_state = state->input_stream;
+  state->input_stream = start_state;
+  HParseResult *r2 = h_do_parse(parsers->p2, state);
+  // TODO(mlp): I'm pretty sure the input stream state should be the post-p1 state in all cases
+  state->input_stream = after_p1_state;
+  // if p2 failed, restore post-p1 state and bail out early
+  if (NULL == r2) {
+    return r1;
+  }
+  size_t r1len = token_length(r1);
+  size_t r2len = token_length(r2);
+  // if both match but p1's text is shorter than p2's, fail
+  if (r1len < r2len) {
+    return NULL;
+  } else {
+    return r1;
+  }
+}
+
+static HParserVtable difference_vt = {
+  .parse = parse_difference,
+};
+
+const HParser* h_difference(const HParser* p1, const HParser* p2) { 
+  HTwoParsers *env = g_new(HTwoParsers, 1);
+  env->p1 = p1; env->p2 = p2;
+  HParser *ret = g_new(HParser, 1);
+  ret->vtable = &difference_vt; ret->env = (void*)env;
+  return ret;
+}