Add string table implementation

Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>

3 files changed, 205 insertions, 0 deletions

diff --git a/include/str_table.h b/include/str_table.h
new file mode 100644
index 0000000..b0d83f1
--- /dev/null
+++ b/include/str_table.h
@@ -0,0 +1,76 @@
+/* SPDX-License-Identifier: GPL-3.0-or-later */
+#ifndef STR_TABLE_H
+#define STR_TABLE_H
+
+typedef struct str_bucket_t {
+	struct str_bucket_t *next;
+	char *str;
+	size_t index;
+} str_bucket_t;
+
+/**
+ * @struct str_table_t
+ *
+ * @brief A data structure that manages incremental, unique IDs for strings
+ *
+ * A string table allocates IDs for strings, and provides fast lookup for ID by
+ * string and string by ID.
+ */
+typedef struct {
+	str_bucket_t **buckets;
+	size_t num_buckets;
+
+	char **strings;
+	size_t num_strings;
+	size_t max_strings;
+} str_table_t;
+
+/**
+ * @brief Initialize a string table
+ *
+ * @memberof str_table_t
+ *
+ * @param size The number of hash table buckets to use internally
+ *
+ * @return Zero on success, -1 on failure (reports error to stderr)
+ */
+int str_table_init(str_table_t *table, size_t size);
+
+/**
+ * @brief Free all memory used by a string table
+ *
+ * @memberof str_table_t
+ *
+ * @param table A pointer to a string table object
+ */
+void str_table_cleanup(str_table_t *table);
+
+/**
+ * @brief Resolve a string to an incremental, unique ID
+ *
+ * @memberof str_table_t
+ *
+ * If the string is passed to this function for the first time, a new ID
+ * is allocated for the string.
+ *
+ * @param table A pointer to a string table object
+ * @param str   A pointer to a string to resolve
+ * @param idx   Returns the unique ID of the string
+ *
+ * @return Zero on success, -1 on failure (reports error to stderr)
+ */
+int str_table_get_index(str_table_t *table, const char *str, size_t *idx);
+
+/**
+ * @brief Resolve a unique ID to the string it represents
+ *
+ * @memberof str_table_t
+ *
+ * @param table A pointer to a string table object
+ * @param index The ID to resolve
+ *
+ * @return A pointer to the string or NULL if it hasn't been added yet
+ */
+const char *str_table_get_string(str_table_t *table, size_t index);
+
+#endif /* STR_TABLE_H */
diff --git a/lib/Makemodule.am b/lib/Makemodule.am
index 17004bc..cd24eae 100644
--- a/lib/Makemodule.am
+++ b/lib/Makemodule.am
@@ -20,6 +20,7 @@ libsquashfs_a_SOURCES += include/frag_reader.h
 libutil_a_SOURCES = lib/util/canonicalize_name.c lib/util/write_retry.c
 libutil_a_SOURCES += lib/util/read_retry.c include/util.h
 libutil_a_SOURCES += lib/util/print_version.c lib/util/mkdir_p.c
+libutil_a_SOURCES += lib/util/str_table.c lib/util/str_table.c
 
 if WITH_GZIP
 libcompress_a_SOURCES += lib/comp/gzip.c
diff --git a/lib/util/str_table.c b/lib/util/str_table.c
new file mode 100644
index 0000000..6363c81
--- /dev/null
+++ b/lib/util/str_table.c
@@ -0,0 +1,128 @@
+/* SPDX-License-Identifier: GPL-3.0-or-later */
+#include <stdint.h>
+#include <stdlib.h>
+#include <string.h>
+#include <stdio.h>
+
+#include "str_table.h"
+
+/* R5 hash function (borrowed from reiserfs) */
+static uint32_t strhash(const char *s)
+{
+	const signed char *str = (const signed char *)s;
+	uint32_t a = 0;
+
+	while (*str != '\0') {
+		a += *str << 4;
+		a += *str >> 4;
+		a *= 11;
+		str++;
+	}
+
+	return a;
+}
+
+static int strings_grow(str_table_t *table)
+{
+	size_t newsz;
+	void *new;
+
+	if (table->num_strings < table->max_strings)
+		return 0;
+
+	newsz = table->max_strings ? (table->max_strings * 2) : 16;
+	new = realloc(table->strings, sizeof(table->strings[0]) * newsz);
+
+	if (new == NULL) {
+		perror("growing string table");
+		return -1;
+	}
+
+	table->strings = new;
+	table->max_strings = newsz;
+	return 0;
+}
+
+int str_table_init(str_table_t *table, size_t size)
+{
+	table->buckets = calloc(size, sizeof(table->buckets[0]));
+	table->num_buckets = size;
+
+	if (table->buckets == NULL) {
+		perror("initializing string table");
+		return -1;
+	}
+
+	return 0;
+}
+
+void str_table_cleanup(str_table_t *table)
+{
+	str_bucket_t *bucket;
+	size_t i;
+
+	for (i = 0; i < table->num_buckets; ++i) {
+		while (table->buckets[i] != NULL) {
+			bucket = table->buckets[i];
+			table->buckets[i] = bucket->next;
+
+			free(bucket->str);
+			free(bucket);
+		}
+	}
+
+	free(table->buckets);
+	free(table->strings);
+	memset(table, 0, sizeof(*table));
+}
+
+int str_table_get_index(str_table_t *table, const char *str, size_t *idx)
+{
+	str_bucket_t *bucket;
+	uint32_t hash;
+	size_t index;
+
+	hash = strhash(str);
+	index = hash % table->num_buckets;
+	bucket = table->buckets[index];
+
+	while (bucket != NULL) {
+		if (strcmp(bucket->str, str) == 0) {
+			*idx = bucket->index;
+			return 0;
+		}
+
+		bucket = bucket->next;
+	}
+
+	if (strings_grow(table))
+		return -1;
+
+	bucket = calloc(1, sizeof(*bucket));
+	if (bucket == NULL)
+		goto fail_oom;
+
+	bucket->str = strdup(str);
+	if (bucket->str == NULL)
+		goto fail_oom;
+
+	bucket->index = table->num_strings;
+	table->strings[table->num_strings++] = bucket->str;
+	*idx = bucket->index;
+
+	bucket->next = table->buckets[index];
+	table->buckets[index] = bucket;
+	return 0;
+fail_oom:
+	free(bucket);
+	perror("allocating hash table bucket");
+	return -1;
+}
+
+const char *str_table_get_string(str_table_t *table, size_t index)
+{
+	if (index >= table->num_strings)
+		return NULL;
+
+	return table->strings[index];
+}