From 2b0fcff7f26d59fed4bcafd1602325122a206c67 Mon Sep 17 00:00:00 2001
From: Pieter Wuille <pieter@wuille.net>
Date: Tue, 17 Mar 2020 20:02:48 -0700
Subject: Make VerifyWitnessProgram use a Span stack

This allows for very cheap transformations on the range of elements that
are to be passed to ExecuteWitnessScript.
---
 src/script/interpreter.cpp | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

(limited to 'src/script')

diff --git a/src/script/interpreter.cpp b/src/script/interpreter.cpp
index 5bf418472a..083022fbdd 100644
--- a/src/script/interpreter.cpp
+++ b/src/script/interpreter.cpp
@@ -1478,9 +1478,9 @@ bool GenericTransactionSignatureChecker<T>::CheckSequence(const CScriptNum& nSeq
 template class GenericTransactionSignatureChecker<CTransaction>;
 template class GenericTransactionSignatureChecker<CMutableTransaction>;
 
-static bool ExecuteWitnessScript(std::vector<valtype>::const_iterator begin, std::vector<valtype>::const_iterator end, const CScript& scriptPubKey, unsigned int flags, SigVersion sigversion, const BaseSignatureChecker& checker, ScriptError* serror)
+static bool ExecuteWitnessScript(const Span<const valtype>& stack_span, const CScript& scriptPubKey, unsigned int flags, SigVersion sigversion, const BaseSignatureChecker& checker, ScriptError* serror)
 {
-    std::vector<valtype> stack{begin, end};
+    std::vector<valtype> stack{stack_span.begin(), stack_span.end()};
 
     // Disallow stack item size > MAX_SCRIPT_ELEMENT_SIZE in witness stack
     for (const valtype& elem : stack) {
@@ -1499,27 +1499,29 @@ static bool ExecuteWitnessScript(std::vector<valtype>::const_iterator begin, std
 static bool VerifyWitnessProgram(const CScriptWitness& witness, int witversion, const std::vector<unsigned char>& program, unsigned int flags, const BaseSignatureChecker& checker, ScriptError* serror)
 {
     CScript scriptPubKey;
+    Span<const valtype> stack = MakeSpan(witness.stack);
 
     if (witversion == 0) {
         if (program.size() == WITNESS_V0_SCRIPTHASH_SIZE) {
             // Version 0 segregated witness program: SHA256(CScript) inside the program, CScript + inputs in witness
-            if (witness.stack.size() == 0) {
+            if (stack.size() == 0) {
                 return set_error(serror, SCRIPT_ERR_WITNESS_PROGRAM_WITNESS_EMPTY);
             }
-            scriptPubKey = CScript(witness.stack.back().begin(), witness.stack.back().end());
+            const valtype& script_bytes = SpanPopBack(stack);
+            scriptPubKey = CScript(script_bytes.begin(), script_bytes.end());
             uint256 hashScriptPubKey;
             CSHA256().Write(&scriptPubKey[0], scriptPubKey.size()).Finalize(hashScriptPubKey.begin());
             if (memcmp(hashScriptPubKey.begin(), program.data(), 32)) {
                 return set_error(serror, SCRIPT_ERR_WITNESS_PROGRAM_MISMATCH);
             }
-            return ExecuteWitnessScript(witness.stack.begin(), witness.stack.end() - 1, scriptPubKey, flags, SigVersion::WITNESS_V0, checker, serror);
+            return ExecuteWitnessScript(stack, scriptPubKey, flags, SigVersion::WITNESS_V0, checker, serror);
         } else if (program.size() == WITNESS_V0_KEYHASH_SIZE) {
             // Special case for pay-to-pubkeyhash; signature + pubkey in witness
-            if (witness.stack.size() != 2) {
+            if (stack.size() != 2) {
                 return set_error(serror, SCRIPT_ERR_WITNESS_PROGRAM_MISMATCH); // 2 items in witness
             }
             scriptPubKey << OP_DUP << OP_HASH160 << program << OP_EQUALVERIFY << OP_CHECKSIG;
-            return ExecuteWitnessScript(witness.stack.begin(), witness.stack.end(), scriptPubKey, flags, SigVersion::WITNESS_V0, checker, serror);
+            return ExecuteWitnessScript(stack, scriptPubKey, flags, SigVersion::WITNESS_V0, checker, serror);
         } else {
             return set_error(serror, SCRIPT_ERR_WITNESS_PROGRAM_WRONG_LENGTH);
         }
-- 
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