parser: improve error reporting

grammar/Makefile

@@ -10,11 +10,8 @@ build:
 build/parser.c: grammar.y | build
 	peg $^ > $@
 
-build/full.c: main.c build/parser.c
-	cat $^ > $@
-
-build/parser: build/full.c
+build/parser: main.c | build/parser.c
 	$(CC) -O3 -o $@ $^
 
-build/parser_debug: build/full.c
+build/parser_debug: main.c
 	$(CC) -O3 -o $@ $^ -DYY_DEBUG

grammar/main.c

@@ -1,10 +1,50 @@
 #include <stdio.h>
 
-int yyparse();
+FILE* input = NULL;
+static int lineno = 0;
+static char* filename= NULL;
+
+#define YY_CTX_LOCAL
+
+#define YY_INPUT(ctx, buf, result, max) {      \
+    int c = getc(input);                       \
+    if (c == '\n') ++lineno;                   \
+    result = (EOF == c) ? 0 : (*(buf) = c, 1); \
+}
+
+#include "build/parser.c"
+
+void yyerror(yycontext* ctx, char* message) {
+    fprintf(stderr, "%s:%d: %s", filename, lineno, message);
+
+    if (ctx->__pos < ctx->__limit || !feof(input)) {
+        // Find the offending line.
+        int pos = ctx->__limit;
+        while (ctx->__pos < pos) {
+	        if (ctx->__buf[pos] == '\n') {
+	            ++pos;
+	            break;
+	        }
+
+	        --pos;
+        }
+
+        ctx->__buf[ctx->__limit] = '\0';
+        fprintf(stderr, "%s", ctx->__buf + pos);
+    }
+
+    fprintf(stderr, "\n");
+}
 
 int main() {
-    if (yyparse() == 0) {
-        fprintf(stderr, "Failed!\n");
+    input = stdin;
+    lineno = 1;
+    filename = "<stdin>";
+
+    yycontext ctx;
+    memset(&ctx, 0, sizeof(yycontext));
+    if (yyparse(&ctx) == 0) {
+        yyerror(&ctx, "syntax error\n");
         return 1;
     }