Fix decoding of negative numeric values, add tests

Author: michal-tomlein

Sources/PostgreSQL/Data/PostgreSQLData+String.swift

@@ -14,7 +14,8 @@ extension String: PostgreSQLDataConvertible {
                     var ndigits: Int16
                     /// How many of the digits are before the decimal point (always add 1)
                     var weight: Int16
-                    /// If 1, this number is negative. Otherwise, positive.
+                    /// If 0x4000, this number is negative. See NUMERIC_NEG in
+                    /// https://github.com/postgres/postgres/blob/master/src/backend/utils/adt/numeric.c
                     var sign: Int16
                     /// The number of sig digits after the decimal place (get rid of trailing 0s)
                     var dscale: Int16
@@ -62,6 +63,10 @@ extension String: PostgreSQLDataConvertible {
                         fractional += String(repeating: "0", count: 4 - string.count) + string
                     }
                 }
+
+                if integer.count == 0 {
+                    integer = "0"
+                }
 
                 if fractional.count > metadata.dscale.bigEndian {
                     /// use the dscale to remove extraneous zeroes at the end of the fractional part
@@ -78,7 +83,7 @@ extension String: PostgreSQLDataConvertible {
                 }
 
                 /// use sign to determine adding a leading `-`
-                if metadata.sign.bigEndian == 1 {
+                if (metadata.sign.bigEndian & 0x4000) != 0 {
                     return "-" + numeric
                 } else {
                     return numeric

Tests/PostgreSQLTests/PostgreSQLConnectionTests.swift

@@ -559,6 +559,36 @@ class PostgreSQLConnectionTests: XCTestCase {
         }
     }
 
+    // https://github.com/vapor/postgresql/pull/111
+    func testNumericDecode() throws {
+        let conn = try PostgreSQLConnection.makeTest()
+
+        let sum = { (value: String) -> PostgreSQLSelectExpression in
+            .expression(.function(.function("SUM", [.expression(.literal(.numeric(value)))])), alias: .identifier("value"))
+        }
+
+        var testValues = ["0.543201203", "1000.1", "10000.1", "42.0001", "42.00001", "10234.543201", "102340567.8"]
+        testValues += testValues.map { "-\($0)" }
+
+        struct NumericString: PostgreSQLTable {
+            let value: String
+        }
+
+        for value in testValues {
+            let result = try conn.select().column(sum(value)).first(decoding: NumericString.self).wait()?.value
+            XCTAssert(result == value)
+        }
+
+        struct NumericDouble: PostgreSQLTable {
+            let value: Double
+        }
+
+        for value in testValues {
+            let result = try conn.select().column(sum(value)).first(decoding: NumericDouble.self).wait()?.value
+            XCTAssert(result == Double(value))
+        }
+    }
+
     static var allTests = [
         ("testBenchmark", testBenchmark),
         ("testVersion", testVersion),
@@ -579,6 +609,7 @@ class PostgreSQLConnectionTests: XCTestCase {
         ("testInvalidDate", testInvalidDate),
         ("testEmptyArray", testEmptyArray),
         ("testZeroNumeric", testZeroNumeric),
+        ("testNumericDecode", testNumericDecode),
     ]
 }