[PATCH 2/4] errors: show carets

[Theophile Ranquet]

* src/locations.c (caret_info): New, persistant information useful
for...
(location_caret): New, print a caret.
(cleanup_caret): Release caret_info cleanly, call it...
* src/main.c (main): Here.
* src/complain.c (error_message): Call location_caret here.
---
 src/complain.c |  3 +++
 src/location.c | 71 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 src/location.h |  5 +++++
 src/main.c     |  2 ++
 4 files changed, 81 insertions(+)

diff --git a/src/complain.c b/src/complain.c
index b063c6b..a0ab3a6 100644
--- a/src/complain.c
+++ b/src/complain.c
@@ -77,8 +77,11 @@ error_message (location *loc,
     if (l < 2 || message[l-2] != ':' || message[l-1] != ' ') {
       putc ('\n', stderr);
       fflush (stderr);
+      if (loc && flag_flag & flag_caret)
+        location_caret (stderr, *loc);
     }
   }
+  fflush (stderr);
 }
 
 /** Wrap error_message() with varargs handling. */
diff --git a/src/location.c b/src/location.c
index fa1b53c..3c3bc6e 100644
--- a/src/location.c
+++ b/src/location.c
@@ -138,6 +138,77 @@ location_print (FILE *out, location loc)
   return res;
 }
 
+struct caret_info
+{
+  FILE* source;
+  size_t line;
+  size_t offset;
+};
+
+static struct caret_info caret_info = { NULL, 1, 0 };
+
+void
+cleanup_caret ()
+{
+  if (caret_info.source)
+    fclose (caret_info.source);
+}
+
+void
+location_caret (FILE *out, location loc)
+{
+  int i;
+  char buf[1024];
+
+  /* FIXME: find a way to support X-file locations, and only open once each
+     file. That would make the procedure future-proof.  */
+  if (! (caret_info.source
+         || (caret_info.source = fopen (loc.start.file, "r")))
+      || loc.start.column > sizeof buf)
+    return;
+
+  /* If the line we want to quote is seekable (the same line as the previous
+     location), just seek it. If it was before, we lost track of it, so
+     return to the start of file.  */
+  if (caret_info.line <= loc.start.line)
+    fseek (caret_info.source, caret_info.offset, SEEK_SET);
+  else
+    {
+      caret_info.line = 1, caret_info.offset = 0;
+      fseek (caret_info.source, caret_info.offset, SEEK_SET);
+    }
+
+  /* Advance to the line's position, keeping track of the offset.  */
+  for (i = caret_info.line; i < loc.start.line; caret_info.offset++)
+    if (fgetc (caret_info.source) == '\n')
+      ++i;
+  caret_info.line = loc.start.line;
+
+  /* Read the actual line.  Don't update the offset, so that we keep a pointer
+     to the start of the line.  */
+  if (fgets (buf, sizeof buf, caret_info.source))
+    {
+      size_t len = strlen (buf);
+      /* The caret of a multiline location ends with the first line.  */
+      int end = loc.start.line != loc.end.line ? len : loc.end.column;
+
+      if (len)
+        {
+          /* Quote the file, by displaying the read line prefixed and
+             suffixed with the indentation, here 1 character.  */
+          buf[len - 1] = '\n';
+          fprintf (out, " %s ", buf);
+
+          /* Print the caret.  */
+          for (i = 1; i < loc.start.column; ++i)
+            fprintf (out, " ");
+          for (; i < end || i == loc.start.column; ++i)
+            fprintf (out, "^");
+        }
+      fprintf (out, "\n");
+    }
+}
+
 void
 boundary_set_from_string (boundary *bound, char *loc_str)
 {
diff --git a/src/location.h b/src/location.h
index 5ebb92e..3bd44be 100644
--- a/src/location.h
+++ b/src/location.h
@@ -102,6 +102,11 @@ void location_compute (location *loc,
    characters.  */
 unsigned location_print (FILE *out, location loc);
 
+void cleanup_caret (void);
+
+/* Output to OUT the line and caret corresponding to location LOC.  */
+void location_caret (FILE *out, location loc);
+
 /* Return -1, 0, 1, depending whether a is before, equal, or
    after b.  */
 static inline int
diff --git a/src/main.c b/src/main.c
index 0396b0f..184d789 100644
--- a/src/main.c
+++ b/src/main.c
@@ -216,5 +216,7 @@ main (int argc, char *argv[])
   timevar_stop (TV_TOTAL);
   timevar_print (stderr);
 
+  cleanup_caret ();
+
   return complaint_issued ? EXIT_FAILURE : EXIT_SUCCESS;
 }
-- 
1.8.0