From a7bfab0e99a8d68b45e35c6a2d8f085719f4bf03 Mon Sep 17 00:00:00 2001
From: Kenny Daniel <platypii@gmail.com>
Date: Mon, 1 Sep 2025 11:24:20 -0700
Subject: [PATCH] Fix high-precision decimal parsing (#116)

---
 src/convert.js                           |  14 ++--
 test/files/issue115decimal.json          |   3 +
 test/files/issue115decimal.metadata.json |  83 +++++++++++++++++++++++
 test/files/issue115decimal.parquet       | Bin 0 -> 557 bytes
 4 files changed, 94 insertions(+), 6 deletions(-)
 create mode 100644 test/files/issue115decimal.json
 create mode 100644 test/files/issue115decimal.metadata.json
 create mode 100644 test/files/issue115decimal.parquet

diff --git a/src/convert.js b/src/convert.js
index be5d897..271d52f 100644
--- a/src/convert.js
+++ b/src/convert.js
@@ -62,7 +62,7 @@ export function convert(data, columnDecoder) {
     const factor = 10 ** -scale
     const arr = new Array(data.length)
     for (let i = 0; i < arr.length; i++) {
-      if (data[0] instanceof Uint8Array) {
+      if (data[i] instanceof Uint8Array) {
         arr[i] = parseDecimal(data[i]) * factor
       } else {
         arr[i] = Number(data[i]) * factor
@@ -155,18 +155,20 @@ export function convert(data, columnDecoder) {
  * @returns {number}
  */
 export function parseDecimal(bytes) {
-  let value = 0
+  if (!bytes.length) return 0
+
+  let value = 0n
   for (const byte of bytes) {
-    value = value * 256 + byte
+    value = value * 256n + BigInt(byte)
   }
 
   // handle signed
   const bits = bytes.length * 8
-  if (value >= 2 ** (bits - 1)) {
-    value -= 2 ** bits
+  if (value >= 2n ** BigInt(bits - 1)) {
+    value -= 2n ** BigInt(bits)
   }
 
-  return value
+  return Number(value)
 }
 
 /**
diff --git a/test/files/issue115decimal.json b/test/files/issue115decimal.json
new file mode 100644
index 0000000..36f350d
--- /dev/null
+++ b/test/files/issue115decimal.json
@@ -0,0 +1,3 @@
+[
+  [-12345.67]
+]
diff --git a/test/files/issue115decimal.metadata.json b/test/files/issue115decimal.metadata.json
new file mode 100644
index 0000000..9f99cfb
--- /dev/null
+++ b/test/files/issue115decimal.metadata.json
@@ -0,0 +1,83 @@
+{
+  "version": 2,
+  "schema": [
+    {
+      "repetition_type": "REQUIRED",
+      "name": "schema",
+      "num_children": 1
+    },
+    {
+      "type": "FIXED_LEN_BYTE_ARRAY",
+      "type_length": 13,
+      "repetition_type": "OPTIONAL",
+      "name": "amount",
+      "converted_type": "DECIMAL",
+      "scale": 2,
+      "precision": 29,
+      "logical_type": {
+        "type": "DECIMAL",
+        "scale": 2,
+        "precision": 29
+      }
+    }
+  ],
+  "num_rows": 1,
+  "row_groups": [
+    {
+      "columns": [
+        {
+          "file_offset": 0,
+          "meta_data": {
+            "type": "FIXED_LEN_BYTE_ARRAY",
+            "encodings": [
+              "PLAIN",
+              "RLE",
+              "RLE_DICTIONARY"
+            ],
+            "path_in_schema": [
+              "amount"
+            ],
+            "codec": "SNAPPY",
+            "num_values": 1,
+            "total_uncompressed_size": 117,
+            "total_compressed_size": 121,
+            "data_page_offset": 33,
+            "dictionary_page_offset": 4,
+            "statistics": {
+              "max": -12345.67,
+              "min": -12345.67,
+              "null_count": 0,
+              "max_value": -12345.67,
+              "min_value": -12345.67
+            },
+            "encoding_stats": [
+              {
+                "page_type": "DICTIONARY_PAGE",
+                "encoding": "PLAIN",
+                "count": 1
+              },
+              {
+                "page_type": "DATA_PAGE",
+                "encoding": "RLE_DICTIONARY",
+                "count": 1
+              }
+            ]
+          }
+        }
+      ],
+      "total_byte_size": 117,
+      "num_rows": 1,
+      "file_offset": 4,
+      "total_compressed_size": 121,
+      "ordinal": 0
+    }
+  ],
+  "key_value_metadata": [
+    {
+      "key": "ARROW:schema",
+      "value": "/////4AAAAAQAAAAAAAKAAwABgAFAAgACgAAAAABBAAMAAAACAAIAAAABAAIAAAABAAAAAEAAAAUAAAAEAAUAAgABgAHAAwAAAAQABAAAAAAAAEHEAAAACAAAAAEAAAAAAAAAAYAAABhbW91bnQAAAgADAAEAAgACAAAAB0AAAACAAAAAAAAAA=="
+    }
+  ],
+  "created_by": "parquet-cpp-arrow version 19.0.1",
+  "metadata_length": 424
+}
diff --git a/test/files/issue115decimal.parquet b/test/files/issue115decimal.parquet
new file mode 100644
index 0000000000000000000000000000000000000000..157948dd516bcd76151f07048365566807024760
GIT binary patch
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literal 0
HcmV?d00001