From 21d69df22cc24c372c2ff0183de675d00b07e6bb Mon Sep 17 00:00:00 2001
From: Yura Lazarev <unisay@gmail.com>
Date: Mon, 13 Jul 2026 15:23:41 +0200
Subject: [PATCH] perf: toStringAs appends digits and reverses once
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table.insert(t, 1, digit) shifts the whole accumulated table on every
loop iteration — quadratic in the digit count (bounded at 32 for Int,
so a constant-factor cost in practice). Append least-significant-first
and reverse the joined string instead; each digit is a single ASCII
byte, so string reverse is safe for every radix up to 36. Measured
1.5-1.6x on 31-digit binary rendering (LuaJIT and PUC 5.1).

Refs purescript-lua/purescript-lua#186.
---
 changelog.d/20260713_150000_tostringas_append.md | 7 +++++++
 src/Data/Int.lua                                 | 9 ++++++---
 2 files changed, 13 insertions(+), 3 deletions(-)
 create mode 100644 changelog.d/20260713_150000_tostringas_append.md

diff --git a/changelog.d/20260713_150000_tostringas_append.md b/changelog.d/20260713_150000_tostringas_append.md
new file mode 100644
index 0000000..076e187
--- /dev/null
+++ b/changelog.d/20260713_150000_tostringas_append.md
@@ -0,0 +1,7 @@
+### Changed
+
+- `toStringAs` accumulates digits by appending and reverses the joined
+  string once, instead of shifting the whole digit table on every
+  iteration with `table.insert(t, 1, …)`. Same output; about 1.5× faster
+  on 31-digit binary rendering (Int digit counts are bounded, so this is a
+  constant-factor win).
diff --git a/src/Data/Int.lua b/src/Data/Int.lua
index f076615..aa9919c 100644
--- a/src/Data/Int.lua
+++ b/src/Data/Int.lua
@@ -61,7 +61,7 @@ return {
   end),
   toStringAs = (function(radix)
     return function(i)
-      local floor, insert = math.floor, table.insert
+      local floor = math.floor
       local n = floor(i)
       if radix == 10 then return tostring(n) end
       -- JS Number.prototype.toString(radix) uses lowercase digits.
@@ -72,12 +72,15 @@ return {
         sign = "-"
         n = -n
       end
+      -- Digits come out least-significant first; append and reverse the
+      -- joined string (each digit is a single ASCII byte, so reverse is
+      -- safe) instead of shifting the table on every insert.
       repeat
         local d = (n % radix) + 1
         n = floor(n / radix)
-        insert(t, 1, digits:sub(d, d))
+        t[#t + 1] = digits:sub(d, d)
       until n == 0
-      return sign .. table.concat(t, "")
+      return sign .. table.concat(t, ""):reverse()
     end
   end),
   quot = (function(x)