From e1bd6c39c32d43693913f0c4b2cf232ddcbbb5f7 Mon Sep 17 00:00:00 2001
From: Manlio Perillo <manlio.perillo@gmail.com>
Date: Sun, 25 Dec 2022 18:42:27 +0100
Subject: [PATCH] parser: improve error reporting

In main.c, define the macro YY_CTX_LOCAL in order to have access to the
parsing context and the macro YY_INPUT in order to have access to the
current line number.  In order to make these macro visible to parser.c,
include parser.c in main.c.

Add the yyerror function for improved error report, showing the current
line and additional context.

Update the Makefile, removing the full.c file.
---
 grammar/Makefile |  7 ++-----
 grammar/main.c   | 46 +++++++++++++++++++++++++++++++++++++++++++---
 2 files changed, 45 insertions(+), 8 deletions(-)

diff --git a/grammar/Makefile b/grammar/Makefile
index c2c7344..98655c7 100644
--- a/grammar/Makefile
+++ b/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
diff --git a/grammar/main.c b/grammar/main.c
index d7f57a9..217abcd 100644
--- a/grammar/main.c
+++ b/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;
     }