From 7d137fd6a9ceb54574481082e9944de168b06b78 Mon Sep 17 00:00:00 2001
From: Boris Kolpackov <boris@codesynthesis.com>
Date: Thu, 22 Sep 2022 13:10:08 +0200
Subject: Add $integer_sequence(<begin>, <end>[, <step>]) function

It returns the list of uint64 integers starting from <begin> (including)
to <end> (excluding) with the specified <step> or 1 if unspecified. For
example:

hdr = foo.hxx bar.hxx baz.hxx
src = foo.cxx bar.cxx baz.cxx

assert ($size($hdr) == $size($src)) "hdr and src expected to be parallel"

for i: $integer_sequence(0, $size($hdr))
{
  h = ($hdr[$i])
  s = ($src[$i])
  ...
}
---
 libbuild2/functions-builtin.cxx   | 31 +++++++++++++++++++++++++++++++
 tests/function/builtin/testscript |  8 ++++++++
 2 files changed, 39 insertions(+)

diff --git a/libbuild2/functions-builtin.cxx b/libbuild2/functions-builtin.cxx
index 5129a05..7000f16 100644
--- a/libbuild2/functions-builtin.cxx
+++ b/libbuild2/functions-builtin.cxx
@@ -123,6 +123,37 @@ namespace build2
 
     f["identity"] += [](value* v) {return move (*v);};
 
+    // $integer_sequence(<begin>, <end>[, <step>])
+    //
+    // Return the list of uint64 integers starting from <begin> (including) to
+    // <end> (excluding) with the specified <step> or 1 if unspecified. If
+    // <begin> is greater than <end>, empty list is returned.
+    //
+    // Note that currently negative numbers are not supported but this could
+    // be handled if required (e.g., by returning int64s in this case).
+    //
+    // Note also that we could improve this by adding a shortcut to get the
+    // indexes of a list (for example, $indexes(<list>) plus potentially a
+    // similar $keys() function for maps).
+    //
+    f["integer_sequence"] += [](value begin, value end, optional<value> step)
+    {
+      uint64_t b (convert<uint64_t> (move (begin)));
+      uint64_t e (convert<uint64_t> (move (end)));
+      uint64_t s (step ? convert<uint64_t> (move (*step)) : 1);
+
+      uint64s r;
+      if (b < e)
+      {
+        r.reserve (static_cast<size_t> ((e - b) / s + 1));
+
+        for (; b < e; b += s)
+          r.push_back (static_cast<size_t> (b));
+      }
+
+      return r;
+    };
+
     // string
     //
     f["string"] += [](bool b) {return b ? "true" : "false";};
diff --git a/tests/function/builtin/testscript b/tests/function/builtin/testscript
index 02c73ee..bbfd4e5 100644
--- a/tests/function/builtin/testscript
+++ b/tests/function/builtin/testscript
@@ -77,6 +77,14 @@
   $* <'print $type($identity(abc))'            >''         : untyped
 }
 
+: integer-sequence
+:
+{
+  $* <'print $integer_sequence(1, 3)'    >'1 2'     : basics
+  $* <'print $integer_sequence(1, 0)'    >''        : empty
+  $* <'print $integer_sequence(0, 8, 2)' >'0 2 4 6' : step
+}
+
 : string
 :
 {
-- 
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