From 88113125716c50ce4deb864041840d53a567554c Mon Sep 17 00:00:00 2001
From: Ava Chow <github@achow101.com>
Date: Mon, 15 Jan 2024 17:08:47 -0500
Subject: [PATCH] script/parsing: Allow Const to not skip the found constant

When parsing a descriptor, it is useful to be able to check whether a
string begins with a substring without consuming that substring as
another function such as Func() will be used later which requires that
substring to be present at the beginning.

Specifically, for MuSig2, this modified Const will be used to determine
whether a an expression begins with "musig(" before a subsequent
Func("musig", ...) is used.
---
 src/script/parsing.cpp  |  4 ++--
 src/script/parsing.h    |  4 ++--
 src/test/util_tests.cpp | 16 +++++++++++++++-
 3 files changed, 19 insertions(+), 5 deletions(-)

diff --git a/src/script/parsing.cpp b/src/script/parsing.cpp
index 254ced6f0b..dd79f67e58 100644
--- a/src/script/parsing.cpp
+++ b/src/script/parsing.cpp
@@ -12,10 +12,10 @@
 
 namespace script {
 
-bool Const(const std::string& str, std::span<const char>& sp)
+bool Const(const std::string& str, std::span<const char>& sp, bool skip)
 {
     if ((size_t)sp.size() >= str.size() && std::equal(str.begin(), str.end(), sp.begin())) {
-        sp = sp.subspan(str.size());
+        if (skip) sp = sp.subspan(str.size());
         return true;
     }
     return false;
diff --git a/src/script/parsing.h b/src/script/parsing.h
index 8986aa57d0..462a42e7b5 100644
--- a/src/script/parsing.h
+++ b/src/script/parsing.h
@@ -13,10 +13,10 @@ namespace script {
 
 /** Parse a constant.
  *
- * If sp's initial part matches str, sp is updated to skip that part, and true is returned.
+ * If sp's initial part matches str, sp is optionally updated to skip that part, and true is returned.
  * Otherwise sp is unmodified and false is returned.
  */
-bool Const(const std::string& str, std::span<const char>& sp);
+bool Const(const std::string& str, std::span<const char>& sp, bool skip = true);
 
 /** Parse a function call.
  *
diff --git a/src/test/util_tests.cpp b/src/test/util_tests.cpp
index 05d2c914c6..8016c2d8c5 100644
--- a/src/test/util_tests.cpp
+++ b/src/test/util_tests.cpp
@@ -1137,13 +1137,24 @@ BOOST_AUTO_TEST_CASE(test_script_parsing)
     BOOST_CHECK(success);
     BOOST_CHECK_EQUAL(SpanToStr(sp), "MilkToastHoney");
 
+    success = Const("Milk", sp, /*skip=*/false);
+    BOOST_CHECK(success);
+    BOOST_CHECK_EQUAL(SpanToStr(sp), "MilkToastHoney");
+
     success = Const("Milk", sp);
     BOOST_CHECK(success);
     BOOST_CHECK_EQUAL(SpanToStr(sp), "ToastHoney");
 
+    success = Const("Bread", sp, /*skip=*/false);
+    BOOST_CHECK(!success);
+
     success = Const("Bread", sp);
     BOOST_CHECK(!success);
 
+    success = Const("Toast", sp, /*skip=*/false);
+    BOOST_CHECK(success);
+    BOOST_CHECK_EQUAL(SpanToStr(sp), "ToastHoney");
+
     success = Const("Toast", sp);
     BOOST_CHECK(success);
     BOOST_CHECK_EQUAL(SpanToStr(sp), "Honey");
@@ -1151,10 +1162,13 @@ BOOST_AUTO_TEST_CASE(test_script_parsing)
     success = Const("Honeybadger", sp);
     BOOST_CHECK(!success);
 
+    success = Const("Honey", sp, /*skip=*/false);
+    BOOST_CHECK(success);
+    BOOST_CHECK_EQUAL(SpanToStr(sp), "Honey");
+
     success = Const("Honey", sp);
     BOOST_CHECK(success);
     BOOST_CHECK_EQUAL(SpanToStr(sp), "");
-
     // Func(...): parse a function call, update span to argument if successful
     input = "Foo(Bar(xy,z()))";
     sp = input;