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	Add helper structure for reading from and writing into buffers - libgrapheme - …
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	---
	commit 563eb65bfbaa4f27c77d73ae81b51882c916993d
	parent 6d0595242a027c1fcb06136e632f6d727388c4ec
	Author: Laslo Hunhold <[email protected]>
	Date: Wed, 21 Sep 2022 20:11:55 +0200

	Add helper structure for reading from and writing into buffers

	The logic behind the input and output buffers is quite intricate
	and leads to numerous subtle bugs that are best handled with a
	refactoring using an abstraction layer that hides most of the
	gory details.

	The Herodotus reader/writer elegantly does all the magic in the
	background, allowing us to focus on the algorithms in the front
	instead. This especially helps with handling NUL-terminated strings,
	as we are guaranteed not to accidentally read too far.

	Signed-off-by: Laslo Hunhold <[email protected]>

	Diffstat:
	M src/util.c \| 240 +++++++++++++++++++++++++++++…
	M src/util.h \| 64 +++++++++++++++++++++++++++++…

	2 files changed, 304 insertions(+), 0 deletions(-)
	---
	diff --git a/src/util.c b/src/util.c
	@@ -7,6 +7,246 @@
	#include "../grapheme.h"
	#include "util.h"

	+void
	+herodotus_reader_init(HERODOTUS_READER *r, enum herodotus_type type,
	+ const void *src, size_t srclen)
	+{
	+ size_t i;
	+
	+ r->type = type;
	+ r->src = src;
	+ r->srclen = srclen;
	+ r->off = 0;
	+ r->terminated_by_null = false;
	+
	+ for (i = 0; i < LEN(r->soft_limit); i++) {
	+ r->soft_limit[i] = SIZE_MAX;
	+ }
	+}
	+
	+void
	+herodotus_reader_copy(const HERODOTUS_READER src, HERODOTUS_READER dest)
	+{
	+ size_t i;
	+
	+ dest->type = src->type;
	+ dest->src = src->src;
	+ dest->srclen = src->srclen;
	+ dest->off = src->off;
	+ dest->terminated_by_null = src->terminated_by_null;
	+
	+ for (i = 0; i < LEN(src->soft_limit); i++) {
	+ dest->soft_limit[i] = src->soft_limit[i];
	+ }
	+}
	+
	+void
	+herodotus_reader_push_advance_limit(HERODOTUS_READER *r, size_t count)
	+{
	+ size_t i;
	+
	+ for (i = LEN(r->soft_limit) - 1; i >= 1; i--) {
	+ r->soft_limit[i] = r->soft_limit[i - 1];
	+ }
	+ r->soft_limit[0] = r->off + count;
	+}
	+
	+void
	+herodotus_reader_pop_limit(HERODOTUS_READER *r)
	+{
	+ size_t i;
	+
	+ for (i = 0; i < LEN(r->soft_limit) - 1; i++) {
	+ r->soft_limit[i] = r->soft_limit[i + 1];
	+ }
	+ r->soft_limit[LEN(r->soft_limit) - 1] = SIZE_MAX;
	+}
	+
	+size_t
	+herodotus_reader_next_word_break(const HERODOTUS_READER *r)
	+{
	+ if (r->type == HERODOTUS_TYPE_CODEPOINT) {
	+ return grapheme_next_word_break(
	+ (const uint_least32_t *)(r->src) + r->off,
	+ MIN(r->srclen, r->soft_limit[0]) - r->off);
	+ } else { /* r->type == HERODOTUS_TYPE_UTF8 */
	+ return grapheme_next_word_break_utf8(
	+ (const char *)(r->src) + r->off,
	+ MIN(r->srclen, r->soft_limit[0]) - r->off);
	+ }
	+}
	+
	+size_t
	+herodotus_reader_next_codepoint_break(const HERODOTUS_READER *r)
	+{
	+ if (r->type == HERODOTUS_TYPE_CODEPOINT) {
	+ return (r->off < MIN(r->srclen, r->soft_limit[0])) ? 1 : 0;
	+ } else { /* r->type == HERODOTUS_TYPE_UTF8 */
	+ return grapheme_decode_utf8(
	+ (const char *)(r->src) + r->off,
	+ MIN(r->srclen, r->soft_limit[0]) - r->off, NULL);
	+ }
	+}
	+
	+enum herodotus_status
	+herodotus_read_codepoint(HERODOTUS_READER r, bool advance, uint_least32_t cp)
	+{
	+ size_t ret;
	+
	+ if (r->terminated_by_null \|\| r->off >= r->srclen \|\| r->src == NULL) {
	+ *cp = GRAPHEME_INVALID_CODEPOINT;
	+ return HERODOTUS_STATUS_END_OF_BUFFER;
	+ }
	+
	+ if (r->off >= r->soft_limit[0]) {
	+ *cp = GRAPHEME_INVALID_CODEPOINT;
	+ return HERODOTUS_STATUS_SOFT_LIMIT_REACHED;
	+ }
	+
	+ if (r->type == HERODOTUS_TYPE_CODEPOINT) {
	+ cp = ((const uint_least32_t )(r->src))[r->off++];
	+ } else { /* r->type == HERODOTUS_TYPE_UTF8 */
	+ ret = grapheme_decode_utf8((const char *)r->src + r->off,
	+ MIN(r->srclen, r->soft_limit[0]) -
	+ r->off, cp);
	+
	+ if (unlikely(r->srclen == SIZE_MAX && *cp == 0)) {
	+ /*
	+ * We encountered a NUL-byte. Don't increment
	+ * offset and return as if the buffer had ended
	+ * here all along
	+ */
	+ r->terminated_by_null = true;
	+ return HERODOTUS_STATUS_END_OF_BUFFER;
	+ }
	+
	+ if (r->off + ret > MIN(r->srclen, r->soft_limit[0])) {
	+ /*
	+ * we want more than we have; instead of
	+ * returning garbage we terminate here.
	+ */
	+ return HERODOTUS_STATUS_END_OF_BUFFER;
	+ }
	+
	+ /*
	+ * Increase offset which we now know won't surpass
	+ * the limits, unless we got told otherwise
	+ */
	+ if (advance) {
	+ r->off += ret;
	+ }
	+ }
	+
	+ return HERODOTUS_STATUS_SUCCESS;
	+}
	+
	+void
	+herodotus_writer_init(HERODOTUS_WRITER *w, enum herodotus_type type,
	+ void *dest, size_t destlen)
	+{
	+ w->type = type;
	+ w->dest = dest;
	+ w->destlen = destlen;
	+ w->off = 0;
	+ w->first_unwritable_offset = SIZE_MAX;
	+}
	+
	+void
	+herodotus_writer_nul_terminate(HERODOTUS_WRITER *w)
	+{
	+ if (w->dest == NULL) {
	+ return;
	+ }
	+
	+ if (w->off < w->destlen) {
	+ /* We still have space in the buffer. Simply use it */
	+ if (w->type == HERODOTUS_TYPE_CODEPOINT) {
	+ ((uint_least32_t *)(w->dest))[w->off] = 0;
	+ } else { /* w->type == HERODOTUS_TYPE_UTF8 */
	+ ((char *)(w->dest))[w->off] = '\0';
	+ }
	+ } else if (w->first_unwritable_offset < w->destlen) {
	+ /*
	+ * There is no more space in the buffer. However,
	+ * we have noted down the first offset we couldn't
	+ * use to write into the buffer and it's smaller than
	+ * destlen. Thus we bailed writing into the
	+ * destination when a multibyte-codepoint couldn't be
	+ * written. So the last "real" byte might be at
	+ * destlen-4, destlen-3, destlen-2 or destlen-1
	+ * (the last case meaning truncation).
	+ */
	+ if (w->type == HERODOTUS_TYPE_CODEPOINT) {
	+ ((uint_least32_t *)(w->dest))
	+ [w->first_unwritable_offset] = 0;
	+ } else { /* w->type == HERODOTUS_TYPE_UTF8 */
	+ ((char *)(w->dest))[w->first_unwritable_offset] = '\0';
	+ }
	+ } else {
	+ /*
	+ * In this case, there is no more space in the buffer and
	+ * the last unwritable offset is larger than
	+ * or equal to the destination buffer length. This means
	+ * that we are forced to simply write into the last
	+ * byte.
	+ */
	+ if (w->type == HERODOTUS_TYPE_CODEPOINT) {
	+ ((uint_least32_t *)(w->dest))
	+ [w->destlen - 1] = 0;
	+ } else { /* w->type == HERODOTUS_TYPE_UTF8 */
	+ ((char *)(w->dest))[w->destlen - 1] = '\0';
	+ }
	+ }
	+
	+ /* w->off is not incremented in any case */
	+}
	+
	+size_t
	+herodotus_writer_number_written(HERODOTUS_WRITER *w)
	+{
	+ return w->off;
	+}
	+
	+void
	+herodotus_write_codepoint(HERODOTUS_WRITER *w, uint_least32_t cp)
	+{
	+ size_t ret;
	+
	+ /*
	+ * This function will always faithfully say how many codepoints
	+ * were written, even if the buffer ends. This is used to enable
	+ * truncation detection.
	+ */
	+ if (w->type == HERODOTUS_TYPE_CODEPOINT) {
	+ if (w->dest != NULL && w->off < w->destlen) {
	+ ((uint_least32_t *)(w->dest))[w->off] = cp;
	+ }
	+
	+ w->off += 1;
	+ } else { /* w->type == HERODOTUS_TYPE_UTF8 */
	+ /*
	+ * First determine how many bytes we need to encode the
	+ * codepoint
	+ */
	+ ret = grapheme_encode_utf8(cp, NULL, 0);
	+
	+ if (w->dest != NULL && w->off + ret < w->destlen) {
	+ /* we still have enough room in the buffer */
	+ grapheme_encode_utf8(cp, (char *)(w->dest) +
	+ w->off, w->destlen - w->off);
	+ } else if (w->first_unwritable_offset == SIZE_MAX) {
	+ /*
	+ * the first unwritable offset has not been
	+ * noted down, so this is the first time we can't
	+ * write (completely) to an offset
	+ */
	+ w->first_unwritable_offset = w->off;
	+ }
	+
	+ w->off += ret;
	+ }
	+}
	+
	inline size_t
	get_codepoint(const void str, size_t len, size_t offset, uint_least32_t cp)
	{
	diff --git a/src/util.h b/src/util.h
	@@ -2,12 +2,16 @@
	#ifndef UTIL_H
	#define UTIL_H

	+#include <stdbool.h>
	#include <stddef.h>
	#include <stdint.h>

	#include "../gen/types.h"
	#include "../grapheme.h"

	+#undef MIN
	+#define MIN(x,y) ((x) < (y) ? (x) : (y))
	+#undef LEN
	#define LEN(x) (sizeof(x) / sizeof(*(x)))

	#undef likely
	@@ -25,6 +29,66 @@
	#define unlikely(expr) (expr)
	#endif

	+/*
	+ * Herodotus, the ancient greek historian and geographer,
	+ * was criticized for including legends and other fantastic
	+ * accounts into his works, among others by his contemporary
	+ * Thucydides.
	+ *
	+ * The Herodotus readers and writers are tailored towards the needs
	+ * of the library interface, doing all the dirty work behind the
	+ * scenes. While the reader is relatively faithful in his accounts,
	+ * the Herodotus writer will never fail and always claim to write the
	+ * data. Internally, it only writes as much as it can, and will simply
	+ * keep account of the rest. This way, we can properly signal truncation.
	+ *
	+ * In this sense, explaining the naming, the writer is always a bit
	+ * inaccurate in his accounts.
	+ *
	+ */
	+enum herodotus_status {
	+ HERODOTUS_STATUS_SUCCESS,
	+ HERODOTUS_STATUS_END_OF_BUFFER,
	+ HERODOTUS_STATUS_SOFT_LIMIT_REACHED,
	+};
	+
	+enum herodotus_type {
	+ HERODOTUS_TYPE_CODEPOINT,
	+ HERODOTUS_TYPE_UTF8,
	+};
	+
	+typedef struct herodotus_reader {
	+ enum herodotus_type type;
	+ const void *src;
	+ size_t srclen;
	+ size_t off;
	+ bool terminated_by_null;
	+ size_t soft_limit[10];
	+} HERODOTUS_READER;
	+
	+typedef struct herodotus_writer {
	+ enum herodotus_type type;
	+ void *dest;
	+ size_t destlen;
	+ size_t off;
	+ size_t first_unwritable_offset;
	+} HERODOTUS_WRITER;
	+
	+void herodotus_reader_init(HERODOTUS_READER *, enum herodotus_type,
	+ const void *, size_t);
	+void herodotus_reader_copy(const HERODOTUS_READER , HERODOTUS_READER );
	+void herodotus_reader_push_advance_limit(HERODOTUS_READER *, size_t);
	+void herodotus_reader_pop_limit(HERODOTUS_READER *);
	+size_t herodotus_reader_next_word_break(const HERODOTUS_READER *);
	+size_t herodotus_reader_next_codepoint_break(const HERODOTUS_READER *);
	+enum herodotus_status herodotus_read_codepoint(HERODOTUS_READER *, bool, uint_…
	+
	+void herodotus_writer_init(HERODOTUS_WRITER , enum herodotus_type, void ,
	+ size_t);
	+void herodotus_writer_nul_terminate(HERODOTUS_WRITER *);
	+size_t herodotus_writer_number_written(HERODOTUS_WRITER *);
	+void herodotus_write_codepoint(HERODOTUS_WRITER *, uint_least32_t);
	+
	size_t get_codepoint(const void , size_t, size_t, uint_least32_t );
	size_t get_codepoint_utf8(const void , size_t, size_t, uint_least32_t );