From f28d03c0a335deb9f7b334cd137b077e041635f3 Mon Sep 17 00:00:00 2001
From: Wouter van Oortmerssen <aardappel@gmail.com>
Date: Tue, 15 Oct 2024 12:28:32 -0700
Subject: [PATCH] slice(): better checking and clearer error for going past the
 end

---
 dev/src/builtins.cpp | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/dev/src/builtins.cpp b/dev/src/builtins.cpp
index 79873c83..8629174b 100644
--- a/dev/src/builtins.cpp
+++ b/dev/src/builtins.cpp
@@ -364,9 +364,11 @@ nfr("slice", "xs,start,size", "A]*II", "A]1",
     [](StackPtr &, VM &vm, Value &l, Value &s, Value &e) {
         auto size = e.ival();
         auto start = s.ival();
-        if (size < 0) size = l.vval()->len - start;
-        if (start < 0 || start + size > l.vval()->len)
-            vm.BuiltinError("slice: values out of range");
+        if (size < 0) size = std::max((iint)0, l.vval()->len - start);
+        if (start < 0)
+            vm.BuiltinError(cat("slice: start cannot be negative: ", start));
+        if (start + size > l.vval()->len)
+            vm.BuiltinError(cat("slice: range extends beyond the end: ", start + size, " > ", l.vval()->len));
         auto nv = (LVector *)vm.NewVec(0, size, l.vval()->tti);
         nv->Append(vm, l.vval(), start, size);
         return Value(nv);