Allow setting floating point precision in JSON.

Fixes #557.

encoder_json.go

@@ -21,6 +21,9 @@ func init() {
 	json.JSONMarshalFunc = func(v interface{}) ([]byte, error) {
 		return InterfaceMarshalFunc(v)
 	}
+	json.FloatingPointPrecisionFunc = func() int {
+		return FloatingPointPrecision
+	}
 }
 
 func appendJSON(dst []byte, j []byte) []byte {

globals.go

@@ -151,6 +151,11 @@ var (
 	// TriggerLevelWriterBufferReuseLimit is a limit in bytes that a buffer is dropped
 	// from the TriggerLevelWriter buffer pool if the buffer grows above the limit.
 	TriggerLevelWriterBufferReuseLimit = 64 * 1024
+
+	// FloatingPointPrecision, if set to a value other than -1, controls the number
+	// of digits when formatting float numbers in JSON. See strconv.FormatFloat for
+	// more details.
+	FloatingPointPrecision = -1
 )
 
 var (

internal/json/base.go

@@ -1,13 +1,17 @@
 package json
 
-// JSONMarshalFunc is used to marshal interface to JSON encoded byte slice.
-// Making it package level instead of embedded in Encoder brings
+// Making the following variables package level instead of embedded in Encoder brings
 // some extra efforts at importing, but avoids value copy when the functions
 // of Encoder being invoked.
-// DO REMEMBER to set this variable at importing, or
+// DO REMEMBER to set these variables at importing, or
 // you might get a nil pointer dereference panic at runtime.
+
+// JSONMarshalFunc is used to marshal interface to JSON encoded byte slice.
 var JSONMarshalFunc func(v interface{}) ([]byte, error)
 
+// FloatingPointPrecisionFunc is used to obtain the floating point formatting precision.
+var FloatingPointPrecisionFunc func() int = func() int { return -1 }
+
 type Encoder struct{}
 
 // AppendKey appends a new key to the output JSON.

internal/json/types.go

@@ -311,16 +311,20 @@ func appendFloat(dst []byte, val float64, bitSize int) []byte {
 	case math.IsInf(val, -1):
 		return append(dst, `"-Inf"`...)
 	}
+	precision := FloatingPointPrecisionFunc()
 	// convert as if by es6 number to string conversion
 	// see also https://cs.opensource.google/go/go/+/refs/tags/go1.20.3:src/encoding/json/encode.go;l=573
 	strFmt := byte('f')
-	// Use float32 comparisons for underlying float32 value to get precise cutoffs right.
-	if abs := math.Abs(val); abs != 0 {
-		if bitSize == 64 && (abs < 1e-6 || abs >= 1e21) || bitSize == 32 && (float32(abs) < 1e-6 || float32(abs) >= 1e21) {
-			strFmt = 'e'
+	// If precision is set to a value other than -1, we always just format the float using that precision.
+	if precision == -1 {
+		// Use float32 comparisons for underlying float32 value to get precise cutoffs right.
+		if abs := math.Abs(val); abs != 0 {
+			if bitSize == 64 && (abs < 1e-6 || abs >= 1e21) || bitSize == 32 && (float32(abs) < 1e-6 || float32(abs) >= 1e21) {
+				strFmt = 'e'
+			}
 		}
 	}
-	dst = strconv.AppendFloat(dst, val, strFmt, -1, bitSize)
+	dst = strconv.AppendFloat(dst, val, strFmt, precision, bitSize)
 	if strFmt == 'e' {
 		// Clean up e-09 to e-9
 		n := len(dst)