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+#ifndef COMMIT_SLAB_H
+#define COMMIT_SLAB_H
+
+/*
+ * define_commit_slab(slabname, elemtype) creates boilerplate code to define
+ * a new struct (struct slabname) that is used to associate a piece of data
+ * of elemtype to commits, and a few functions to use that struct.
+ *
+ * After including this header file, using:
+ *
+ * define_commit_slab(indegee, int);
+ *
+ * will let you call the following functions:
+ *
+ * - int *indegree_at(struct indegree *, struct commit *);
+ *
+ * This function locates the data associated with the given commit in
+ * the indegree slab, and returns the pointer to it. The location to
+ * store the data is allocated as necessary.
+ *
+ * - int *indegree_peek(struct indegree *, struct commit *);
+ *
+ * This function is similar to indegree_at(), but it will return NULL
+ * until a call to indegree_at() was made for the commit.
+ *
+ * - void init_indegree(struct indegree *);
+ * void init_indegree_with_stride(struct indegree *, int);
+ *
+ * Initializes the indegree slab that associates an array of integers
+ * to each commit. 'stride' specifies how big each array is. The slab
+ * that is initialized by the variant without "_with_stride" associates
+ * each commit with an array of one integer.
+ *
+ * - void clear_indegree(struct indegree *);
+ *
+ * Empties the slab. The slab can be reused with the same stride
+ * without calling init_indegree() again or can be reconfigured to a
+ * different stride by calling init_indegree_with_stride().
+ *
+ * Call this function before the slab falls out of scope to avoid
+ * leaking memory.
+ */
+
+/* allocate ~512kB at once, allowing for malloc overhead */
+#ifndef COMMIT_SLAB_SIZE
+#define COMMIT_SLAB_SIZE (512*1024-32)
+#endif
+
+#define MAYBE_UNUSED __attribute__((__unused__))
+
+#define define_commit_slab(slabname, elemtype) \
+ \
+struct slabname { \
+ unsigned slab_size; \
+ unsigned stride; \
+ unsigned slab_count; \
+ elemtype **slab; \
+}; \
+static int stat_ ##slabname## realloc; \
+ \
+static MAYBE_UNUSED void init_ ##slabname## _with_stride(struct slabname *s, \
+ unsigned stride) \
+{ \
+ unsigned int elem_size; \
+ if (!stride) \
+ stride = 1; \
+ s->stride = stride; \
+ elem_size = sizeof(elemtype) * stride; \
+ s->slab_size = COMMIT_SLAB_SIZE / elem_size; \
+ s->slab_count = 0; \
+ s->slab = NULL; \
+} \
+ \
+static MAYBE_UNUSED void init_ ##slabname(struct slabname *s) \
+{ \
+ init_ ##slabname## _with_stride(s, 1); \
+} \
+ \
+static MAYBE_UNUSED void clear_ ##slabname(struct slabname *s) \
+{ \
+ int i; \
+ for (i = 0; i < s->slab_count; i++) \
+ free(s->slab[i]); \
+ s->slab_count = 0; \
+ free(s->slab); \
+ s->slab = NULL; \
+} \
+ \
+static MAYBE_UNUSED elemtype *slabname## _at_peek(struct slabname *s, \
+ const struct commit *c, \
+ int add_if_missing) \
+{ \
+ int nth_slab, nth_slot; \
+ \
+ nth_slab = c->index / s->slab_size; \
+ nth_slot = c->index % s->slab_size; \
+ \
+ if (s->slab_count <= nth_slab) { \
+ int i; \
+ if (!add_if_missing) \
+ return NULL; \
+ REALLOC_ARRAY(s->slab, nth_slab + 1); \
+ stat_ ##slabname## realloc++; \
+ for (i = s->slab_count; i <= nth_slab; i++) \
+ s->slab[i] = NULL; \
+ s->slab_count = nth_slab + 1; \
+ } \
+ if (!s->slab[nth_slab]) { \
+ if (!add_if_missing) \
+ return NULL; \
+ s->slab[nth_slab] = xcalloc(s->slab_size, \
+ sizeof(**s->slab) * s->stride); \
+ } \
+ return &s->slab[nth_slab][nth_slot * s->stride]; \
+} \
+ \
+static MAYBE_UNUSED elemtype *slabname## _at(struct slabname *s, \
+ const struct commit *c) \
+{ \
+ return slabname##_at_peek(s, c, 1); \
+} \
+ \
+static MAYBE_UNUSED elemtype *slabname## _peek(struct slabname *s, \
+ const struct commit *c) \
+{ \
+ return slabname##_at_peek(s, c, 0); \
+} \
+ \
+static int stat_ ##slabname## realloc
+
+/*
+ * Note that this seemingly redundant second declaration is required
+ * to allow a terminating semicolon, which makes instantiations look
+ * like function declarations. I.e., the expansion of
+ *
+ * define_commit_slab(indegree, int);
+ *
+ * ends in 'static int stat_indegreerealloc;'. This would otherwise
+ * be a syntax error according (at least) to ISO C. It's hard to
+ * catch because GCC silently parses it by default.
+ */
+
+/*
+ * Statically initialize a commit slab named "var". Note that this
+ * evaluates "stride" multiple times! Example:
+ *
+ * struct indegree indegrees = COMMIT_SLAB_INIT(1, indegrees);
+ *
+ */
+#define COMMIT_SLAB_INIT(stride, var) { \
+ COMMIT_SLAB_SIZE / sizeof(**((var).slab)) / (stride), \
+ (stride), 0, NULL \
+}
+
+#endif /* COMMIT_SLAB_H */