extrakeys: add secp256k1_pubkey_sort

include/secp256k1_extrakeys.h

@@ -240,6 +240,21 @@ SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_keypair_xonly_tweak_add
     const unsigned char *tweak32
 ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
 
+/** Sort public keys using lexicographic order of their compressed
+ *  serialization.
+ *
+ *  Returns: 0 if the arguments are invalid. 1 otherwise.
+ *
+ *  Args:     ctx: pointer to a context object
+ *  In:   pubkeys: array of pointers to pubkeys to sort
+ *      n_pubkeys: number of elements in the pubkeys array
+ */
+SECP256K1_API int secp256k1_pubkey_sort(
+    const secp256k1_context *ctx,
+    const secp256k1_pubkey **pubkeys,
+    size_t n_pubkeys
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2);
+
 #ifdef __cplusplus
 }
 #endif

src/modules/extrakeys/Makefile.am.include

@@ -2,3 +2,5 @@ include_HEADERS += include/secp256k1_extrakeys.h
 noinst_HEADERS += src/modules/extrakeys/tests_impl.h
 noinst_HEADERS += src/modules/extrakeys/tests_exhaustive_impl.h
 noinst_HEADERS += src/modules/extrakeys/main_impl.h
+noinst_HEADERS += src/modules/extrakeys/hsort.h
+noinst_HEADERS += src/modules/extrakeys/hsort_impl.h

src/modules/extrakeys/hsort.h

@@ -0,0 +1,22 @@
+/***********************************************************************
+ * Copyright (c) 2021 Russell O'Connor, Jonas Nick                     *
+ * Distributed under the MIT software license, see the accompanying    *
+ * file COPYING or https://www.opensource.org/licenses/mit-license.php.*
+ ***********************************************************************/
+
+#ifndef SECP256K1_HSORT_H
+#define SECP256K1_HSORT_H
+
+#include <stddef.h>
+#include <string.h>
+
+/* In-place, iterative heapsort with an interface matching glibc's qsort_r. This
+ * is preferred over standard library implementations because they generally
+ * make no guarantee about being fast for malicious inputs.
+ *
+ * See the qsort_r manpage for a description of the interface.
+ */
+static void secp256k1_hsort(void *ptr, size_t count, size_t size,
+                            int (*cmp)(const void *, const void *, void *),
+                            void *cmp_data);
+#endif

src/modules/extrakeys/hsort_impl.h

@@ -0,0 +1,116 @@
+/***********************************************************************
+ * Copyright (c) 2021 Russell O'Connor, Jonas Nick                     *
+ * Distributed under the MIT software license, see the accompanying    *
+ * file COPYING or https://www.opensource.org/licenses/mit-license.php.*
+ ***********************************************************************/
+
+#ifndef SECP256K1_HSORT_IMPL_H
+#define SECP256K1_HSORT_IMPL_H
+
+#include "hsort.h"
+
+/* An array is a heap when, for all non-zero indexes i, the element at index i
+ * compares as less than or equal to the element at index parent(i) = (i-1)/2.
+ */
+
+static SECP256K1_INLINE size_t child1(size_t i) {
+    VERIFY_CHECK(i <= (SIZE_MAX - 1)/2);
+    return 2*i + 1;
+}
+
+static SECP256K1_INLINE size_t child2(size_t i) {
+    VERIFY_CHECK(i <= SIZE_MAX/2 - 1);
+    return child1(i)+1;
+}
+
+static SECP256K1_INLINE void heap_swap64(unsigned char *a, size_t i, size_t j, size_t stride) {
+    unsigned char tmp[64];
+    VERIFY_CHECK(stride <= 64);
+    memcpy(tmp, a + i*stride, stride);
+    memmove(a + i*stride, a + j*stride, stride);
+    memcpy(a + j*stride, tmp, stride);
+}
+
+static SECP256K1_INLINE void heap_swap(unsigned char *a, size_t i, size_t j, size_t stride) {
+    while (64 < stride) {
+        heap_swap64(a + (stride - 64), i, j, 64);
+        stride -= 64;
+    }
+    heap_swap64(a, i, j, stride);
+}
+
+static SECP256K1_INLINE void heap_down(unsigned char *a, size_t i, size_t heap_size, size_t stride,
+                            int (*cmp)(const void *, const void *, void *), void *cmp_data) {
+    while (i < heap_size/2) {
+        VERIFY_CHECK(i <= SIZE_MAX/2 - 1);
+        /* Proof:
+         * i < heap_size/2
+         * i + 1 <= heap_size/2
+         * 2*i + 2 <= heap_size <= SIZE_MAX
+         * 2*i <= SIZE_MAX - 2
+         */
+
+        VERIFY_CHECK(child1(i) < heap_size);
+        /* Proof:
+         * i < heap_size/2
+         * i + 1 <= heap_size/2
+         * 2*i + 2 <= heap_size
+         * 2*i + 1 < heap_size
+         * child1(i) < heap_size
+         */
+
+        /* Let [x] be notation for the contents at a[x*stride].
+         *
+         * If [child1(i)] > [i] and [child2(i)] > [i],
+         *   swap [i] with the larger child to ensure the new parent is larger
+         *   than both children. When [child1(i)] == [child2(i)], swap [i] with
+         *   [child2(i)].
+         * Else if [child1(i)] > [i], swap [i] with [child1(i)].
+         * Else if [child2(i)] > [i], swap [i] with [child2(i)].
+         */
+        if (child2(i) < heap_size
+                && 0 <= cmp(a + child2(i)*stride, a + child1(i)*stride, cmp_data)) {
+            if (0 < cmp(a + child2(i)*stride, a +         i*stride, cmp_data)) {
+                heap_swap(a, i, child2(i), stride);
+                i = child2(i);
+            } else {
+                /* At this point we have [child2(i)] >= [child1(i)] and we have
+                 * [child2(i)] <= [i], and thus [child1(i)] <= [i] which means
+                 * that the next comparison can be skipped. */
+                return;
+            }
+        } else if (0 < cmp(a + child1(i)*stride, a +         i*stride, cmp_data)) {
+            heap_swap(a, i, child1(i), stride);
+            i = child1(i);
+        } else {
+            return;
+        }
+    }
+    /* heap_size/2 <= i
+     * heap_size/2 < i + 1
+     * heap_size < 2*i + 2
+     * heap_size <= 2*i + 1
+     * heap_size <= child1(i)
+     * Thus child1(i) and child2(i) are now out of bounds and we are at a leaf.
+     */
+}
+
+/* In-place heap sort. */
+static void secp256k1_hsort(void *ptr, size_t count, size_t size,
+                            int (*cmp)(const void *, const void *, void *),
+                            void *cmp_data ) {
+    size_t i;
+
+    for(i = count/2; 0 < i; --i) {
+        heap_down(ptr, i-1, count, size, cmp, cmp_data);
+    }
+    for(i = count; 1 < i; --i) {
+        /* Extract the largest value from the heap */
+        heap_swap(ptr, 0, i-1, size);
+
+        /* Repair the heap condition */
+        heap_down(ptr, 0, i-1, size, cmp, cmp_data);
+    }
+}
+
+#endif

src/modules/extrakeys/main_impl.h

@@ -9,6 +9,7 @@
 
 #include "../../../include/secp256k1.h"
 #include "../../../include/secp256k1_extrakeys.h"
+#include "hsort_impl.h"
 #include "../../util.h"
 
 static SECP256K1_INLINE int secp256k1_xonly_pubkey_load(const secp256k1_context* ctx, secp256k1_ge *ge, const secp256k1_xonly_pubkey *pubkey) {
@@ -282,4 +283,38 @@ int secp256k1_keypair_xonly_tweak_add(const secp256k1_context* ctx, secp256k1_ke
     return ret;
 }
 
+/* This struct wraps a const context pointer to satisfy the secp256k1_hsort api
+ * which expects a non-const cmp_data pointer. */
+typedef struct {
+    const secp256k1_context *ctx;
+} secp256k1_pubkey_sort_cmp_data;
+
+static int secp256k1_pubkey_sort_cmp(const void* pk1, const void* pk2, void *cmp_data) {
+    return secp256k1_ec_pubkey_cmp(((secp256k1_pubkey_sort_cmp_data*)cmp_data)->ctx,
+                                     *(secp256k1_pubkey **)pk1,
+                                     *(secp256k1_pubkey **)pk2);
+}
+
+int secp256k1_pubkey_sort(const secp256k1_context* ctx, const secp256k1_pubkey **pubkeys, size_t n_pubkeys) {
+    secp256k1_pubkey_sort_cmp_data cmp_data;
+    VERIFY_CHECK(ctx != NULL);
+    ARG_CHECK(pubkeys != NULL);
+
+    cmp_data.ctx = ctx;
+
+    /* Suppress wrong warning (fixed in MSVC 19.33) */
+    #if defined(_MSC_VER) && (_MSC_VER < 1933)
+    #pragma warning(push)
+    #pragma warning(disable: 4090)
+    #endif
+
+    secp256k1_hsort(pubkeys, n_pubkeys, sizeof(*pubkeys), secp256k1_pubkey_sort_cmp, &cmp_data);
+
+    #if defined(_MSC_VER) && (_MSC_VER < 1933)
+    #pragma warning(pop)
+    #endif
+
+    return 1;
+}
+
 #endif