feat: render diagnostic source excerpts

fec/src/diag.c

@@ -1,9 +1,62 @@
 #include "diag.h"
 
+static unsigned long digits(unsigned long n)
+{
+    unsigned long count=1;
+    while(n>=10UL){n/=10UL;++count;}
+    return count;
+}
+
+static void excerpt(const FeDiags *d, FeLoc loc)
+{
+    const char *src;
+    unsigned long len;
+    unsigned long i=0;
+    unsigned long line=1;
+    unsigned long start;
+    unsigned long end;
+    unsigned long gutter;
+    unsigned long col;
+    char c;
+    if(!d || !d->source || !d->source_len || !loc.line || !loc.col) return;
+    src=d->source;
+    len=d->source_len;
+    while(i<len && line<loc.line){if(src[i++]=='\n')++line;}
+    if(line!=loc.line || i>len) return;
+    start=i;
+    end=start;
+    while(end<len && src[end]!='\n' && src[end]!='\r')++end;
+    gutter=digits(loc.line);
+    fputs("  ",stderr);
+    fprintf(stderr,"%lu | ",loc.line);
+    if(end>start) fwrite(src+start,1,(size_t)(end-start),stderr);
+    fputc('\n',stderr);
+    fputs("  ",stderr);
+    for(i=0;i<gutter;++i) fputc(' ',stderr);
+    fputs(" | ",stderr);
+    col=1;
+    i=start;
+    while(i<end && col<loc.col){
+        c=src[i++];
+        fputc(c=='\t' ? '\t' : ' ',stderr);
+        ++col;
+    }
+    fputs("^\n",stderr);
+}
+
+void fe_diags_init(FeDiags *d, const char *source, unsigned long source_len)
+{
+    d->errors=0;
+    d->warnings=0;
+    d->source=source;
+    d->source_len=source_len;
+}
+
 void fe_diag_error(FeDiags *d, FeLoc loc, const char *msg)
 {
     d->errors++;
     fprintf(stderr, "%s:%lu:%lu: error: %s\n", loc.file ? loc.file : "<source>", loc.line, loc.col, msg);
+    excerpt(d,loc);
 }
 
 void fe_diag_errorf(FeDiags *d, FeLoc loc, const char *msg, const char *arg)
@@ -12,9 +65,16 @@ void fe_diag_errorf(FeDiags *d, FeLoc loc, const char *msg, const char *arg)
     fprintf(stderr, "%s:%lu:%lu: error: ", loc.file ? loc.file : "<source>", loc.line, loc.col);
     fprintf(stderr, msg, arg);
     fputc('\n', stderr);
+    excerpt(d,loc);
 }
 
 void fe_diag_note(FeLoc loc, const char *msg)
 {
     fprintf(stderr, "%s:%lu:%lu: note: %s\n", loc.file ? loc.file : "<source>", loc.line, loc.col, msg);
 }
+
+void fe_diag_note_src(FeDiags *d, FeLoc loc, const char *msg)
+{
+    fe_diag_note(loc,msg);
+    excerpt(d,loc);
+}