From d5d0883ec84653c13c01d35824c95fa3dd682348 Mon Sep 17 00:00:00 2001
From: Daniel Jones <admin@danieljon.es>
Date: Thu, 26 Dec 2019 16:54:07 +1030
Subject: added ability to write to an offset in a file

---
 pages.c | 87 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 pages.h |  1 +
 2 files changed, 88 insertions(+)

diff --git a/pages.c b/pages.c
index 49f95c4..c4172ea 100644
--- a/pages.c
+++ b/pages.c
@@ -200,6 +200,92 @@ deletebytes(const char *file, long offset, size_t bytes)
 	return 1;
 }
 
+int
+writefileatbyte(const char *dest, const char *source, long offset)
+{
+	/*
+	 * write  file 'source' into 'output_dir'/'file' starting at offset 'offset'
+	 * return 1 on success. 0 on failure
+	 */
+
+	char filename[512] = {0};
+	size_t length = 512;
+	strncat(filename, output_dir, length-1);
+	length = length - strlen(output_dir);
+	strncat(filename, dest, length-1);
+
+	FILE *in = fopen(filename, "r");
+	if (!in)
+	{
+		fprintf(stderr, "unable to open file '%s', unrecoverable failure\n", filename);
+		return 0;
+	}
+
+	FILE *tmp = fopen("tmp.tmp", "w");
+	if (!tmp)
+	{
+		fprintf(stderr, "unable to open temp file, unrecoverable failure\n");
+		fclose(in);
+		return 0;
+	}
+
+	FILE *src = fopen(source, "r");
+	if (!src)
+	{
+		fprintf(stderr, "unable to open file '%s', unrecoverable failure\n", source);
+		fclose(in);
+		fclose(tmp);
+		return 0;
+	}
+
+	/*
+	 * read file 'in' until byte 'offset', writing into file 'tmp'
+	 * write file 'src' into 'tmp'
+	 * finish writing file 'in' into 'tmp'
+	 * delete file 'filename'
+	 * rename file 'tmp' 'filename'
+	 */
+
+	char c, t, a;
+	long curpos = 1;
+	while ((c = fgetc(in)) != EOF)
+	{
+		if (curpos == offset)
+		{
+			while ((t = fgetc(src)) != EOF)
+			{
+				/* prevent writing end of line '\n' if the next char is EOF */
+				a = fgetc(src);
+				ungetc(a, src);
+				if (a == EOF && t == '\n')
+					continue;
+				fputc(t, tmp);
+			}
+		}
+		fputc(c, tmp);
+		curpos++;
+	}
+
+	fclose(tmp);
+	fclose(in);
+	fclose(src);
+
+	if (remove(filename) == -1)
+	{
+		fprintf(stderr, "unable to delete file '%s', unrecoverable failure\n", filename);
+		return 0;
+	}
+
+	if (rename("tmp.tmp", filename) == -1)
+	{
+
+		fprintf(stderr, "unable to rename tmp file to '%s', unrecoverable failure\n", filename);
+		return 0;
+	}
+
+	return 1;
+}
+
 int
 frontpage(int flags)
 {
@@ -227,5 +313,6 @@ frontpage(int flags)
 		fprintf(stderr, "unable to generate frontpage\n");
 		return 0;
 	}
+	writefileatbyte(frontpage_index_output, "source.txt", substrpos);
 	return 1;
 }
diff --git a/pages.h b/pages.h
index 907d66e..658f17a 100644
--- a/pages.h
+++ b/pages.h
@@ -25,6 +25,7 @@
 int createfile(const char *file);
 long findstring(const char *file, const char *str);
 int deletebytes(const char *file, long offset, size_t bytes);
+int writefileatbyte(const char *dest, const char *source, long offset);
 
 /* generators (to be put into the pages array) */
 int frontpage(int flags);
-- 
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