From 45de80c852dc0498d0be3149a5aa2865f9501a12 Mon Sep 17 00:00:00 2001
From: Josh Holtrop <jholtrop@gmail.com>
Date: Mon, 23 Apr 2018 23:11:30 -0400
Subject: [PATCH] show source line on error

---
 src/parser/parser.c | 77 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 77 insertions(+)

diff --git a/src/parser/parser.c b/src/parser/parser.c
index e3f71cc..117f257 100644
--- a/src/parser/parser.c
+++ b/src/parser/parser.c
@@ -1,5 +1,9 @@
 #include "parser.h"
 #include <stdio.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <unistd.h>
 
 extern FILE * yyin;
 
@@ -21,10 +25,83 @@ void parse(const char * filename)
     }
 }
 
+static char * read_file(const char * filename, size_t * length)
+{
+    int fd = open(filename, O_RDONLY);
+    if (fd < 0)
+        return NULL;
+    size_t size = (size_t)lseek(fd, 0, SEEK_END);
+    lseek(fd, 0, SEEK_SET);
+    *length = size;
+    size_t n_bytes_read = 0u;
+    char * buf = (char *)malloc(size);
+    if (buf == NULL)
+    {
+        close(fd);
+        return NULL;
+    }
+    for (;;)
+    {
+        off_t rd_size = read(fd, &buf[n_bytes_read], size - n_bytes_read);
+        if (rd_size <= 0)
+            break;
+        n_bytes_read += (size_t)rd_size;
+        if (n_bytes_read >= size)
+            break;
+    }
+    if (n_bytes_read != size)
+    {
+        free(buf);
+        close(fd);
+        return NULL;
+    }
+    close(fd);
+    return buf;
+}
+
+static void display_error_source(int line, int column)
+{
+    if (line < 0)
+        return;
+
+    size_t length = 0u;
+
+    char * file = read_file(input_fname, &length);
+    if (file == NULL)
+        return;
+
+    int search_line = 1;
+    const char * line_begin = NULL;
+    for (size_t i = 0u; i < length; i++)
+    {
+        if (search_line == line)
+        {
+            line_begin = &file[i];
+            search_line++;
+        }
+        if (file[i] == '\n')
+        {
+            if (line_begin != NULL)
+            {
+                file[i] = '\0';
+                fprintf(stderr, "%s\n", line_begin);
+                for (int i = 0; i < column; i++)
+                {
+                    fprintf(stderr, " ");
+                }
+                fprintf(stderr, "^\n");
+                break;
+            }
+            search_line++;
+        }
+    }
+}
+
 void handle_parse_error(const char * str, const YYLTYPE * yylloc)
 {
     fprintf(stderr, "error: %s (line %d, column %d)\n",
             str,
             yylloc->first_line,
             yylloc->first_column);
+    display_error_source(yylloc->first_line, yylloc->first_column);
 }