decimal: add support decimal type in msgpack

This patch provides decimal support for all space operations and as
function return result. Decimal type was introduced in Tarantool 2.2.
See more about decimal type in [1] and [2].

According to BCD encoding/decoding specification sign is encoded by
letters: '0x0a', '0x0c', '0x0e', '0x0f' stands for plus, and '0x0b' and
'0x0d' for minus. Tarantool always uses '0x0c' for plus and '0x0d' for
minus. Implementation in Golang follows the same rule and in all test
samples sign encoded by '0x0d' and '0x0c' for simplification.

Because 'c' used by Tarantool.

To use decimal with github.com/shopspring/decimal in msgpack, import
tarantool/decimal submodule.

1. https://www.tarantool.io/en/doc/latest/book/box/data_model/
2. https://www.tarantool.io/en/doc/latest/dev_guide/internals/msgpack_extensions/#the-decimal-type
3. https://github.com/douglascrockford/DEC64/blob/663f562a5f0621021b98bfdd4693571993316174/dec64_test.c#L62-L104
4. https://github.com/shopspring/decimal/blob/v1.3.1/decimal_test.go#L27-L64
5. https://github.com/tarantool/tarantool/blob/60fe9d14c1c7896aa7d961e4b68649eddb4d2d6c/test/unit/decimal.c#L154-L171

Lua snippet for encoding number to MsgPack representation:

local decimal = require('decimal')

local function mp_encode_dec(num)
    local dec = msgpack.encode(decimal.new(num))
    return dec:gsub('.', function (c)
        return string.format('%02x', string.byte(c))
    end)
end
print(mp_encode_dec(-12.34)) -- 0xd6010201234d

Follows up tarantool/tarantool#692
Part of #96

CHANGELOG.md

@@ -32,6 +32,7 @@ CI and documentation.
 - queue-utube handling (#85)
 - Master discovery (#113)
 - SQL support (#62)
+- Support decimal type in msgpack (#96)
 
 ### Changed
 

Makefile

@@ -70,6 +70,12 @@ test-main:
 	go clean -testcache
 	go test . -v -p 1
 
+.PHONY: test-decimal
+test-decimal:
+	@echo "Running tests in decimal package"
+	go clean -testcache
+	go test ./decimal/ -v -p 1
+
 .PHONY: coverage
 coverage:
 	go clean -testcache

decimal/bcd.go

@@ -0,0 +1,253 @@
+// Package decimal implements methods to encode and decode BCD.
+//
+// BCD (Binary-Coded Decimal) is a sequence of bytes representing decimal
+// digits of the encoded number (each byte has two decimal digits each encoded
+// using 4-bit nibbles), so byte >> 4 is the first digit and byte & 0x0f is the
+// second digit. The leftmost digit in the array is the most significant. The
+// rightmost digit in the array is the least significant.
+//
+// The first byte of the BCD array contains the first digit of the number,
+// represented as follows:
+//
+//     |  4 bits           |  4 bits           |
+//        = 0x                = the 1st digit
+//
+// (The first nibble contains 0 if the decimal number has an even number of
+// digits). The last byte of the BCD array contains the last digit of the
+// number and the final nibble, represented as follows:
+//
+//     |  4 bits           |  4 bits           |
+//        = the last digit    = nibble
+//
+// The final nibble represents the number's sign: 0x0a, 0x0c, 0x0e, 0x0f stand
+// for plus, 0x0b and 0x0d stand for minus.
+//
+// Examples:
+//
+// The decimal -12.34 will be encoded as 0xd6, 0x01, 0x02, 0x01, 0x23, 0x4d:
+//
+// | MP_EXT (fixext 4) | MP_DECIMAL | scale |  1   |  2,3 |  4 (minus) |
+// |        0xd6       |    0x01    | 0x02  | 0x01 | 0x23 | 0x4d       |
+//
+// The decimal 0.000000000000000000000000000000000010 will be encoded as
+// 0xc7, 0x03, 0x01, 0x24, 0x01, 0x0c:
+//
+// | MP_EXT (ext 8) | length | MP_DECIMAL | scale |  1   | 0 (plus) |
+// |      0xc7      |  0x03  |    0x01    | 0x24  | 0x01 | 0x0c     |
+//
+// See also:
+//
+// * MessagePack extensions https://www.tarantool.io/en/doc/latest/dev_guide/internals/msgpack_extensions/
+//
+// * An implementation in C language https://github.com/tarantool/decNumber/blob/master/decPacked.c
+package decimal
+
+import (
+	"fmt"
+	"strings"
+)
+
+const (
+	bytePlus  = byte(0x0c)
+	byteMinus = byte(0x0d)
+)
+
+var isNegative = [256]bool{
+	0x0a: false,
+	0x0b: true,
+	0x0c: false,
+	0x0d: true,
+	0x0e: false,
+	0x0f: false,
+}
+
+// Calculate a number of digits in a buffer with decimal number.
+//
+// Plus, minus, point and leading zeroes do not count.
+// Contains a quirk for a zero - returns 1.
+//
+// Examples (see more examples in tests):
+//
+// - 0.0000000000000001 - 1 digit
+//
+// - 00012.34           - 4 digits
+//
+// - 0.340              - 3 digits
+//
+// - 0                  - 1 digit
+func GetNumberLength(buf string) int {
+	if len(buf) == 0 {
+		return 0
+	}
+	n := 0
+	for _, ch := range []byte(buf) {
+		if ch >= '1' && ch <= '9' {
+			n += 1
+		} else if ch == '0' && n != 0 {
+			n += 1
+		}
+	}
+
+	// Fix a case with a single 0.
+	if n == 0 {
+		n = 1
+	}
+
+	return n
+}
+
+// EncodeStringToBCD converts a string buffer to BCD Packed Decimal.
+//
+// The number is converted to a BCD packed decimal byte array, right aligned in
+// the BCD array, whose length is indicated by the second parameter. The final
+// 4-bit nibble in the array will be a sign nibble, 0x0c for "+" and 0x0d for
+// "-". Unused bytes and nibbles to the left of the number are set to 0. scale
+// is set to the scale of the number (this is the exponent, negated).
+func EncodeStringToBCD(buf string) ([]byte, error) {
+	if len(buf) == 0 {
+		return nil, fmt.Errorf("Length of number is zero")
+	}
+	signByte := bytePlus // By default number is positive.
+	if buf[0] == '-' {
+		signByte = byteMinus
+	}
+
+	// The first nibble should contain 0, if the decimal number has an even
+	// number of digits. Therefore highNibble is false when decimal number
+	// is even.
+	highNibble := true
+	l := GetNumberLength(buf)
+	if l%2 == 0 {
+		highNibble = false
+	}
+	scale := 0 // By default decimal number is integer.
+	var byteBuf []byte
+	for i, ch := range []byte(buf) {
+		// Skip leading zeroes.
+		if (len(byteBuf) == 0) && ch == '0' {
+			continue
+		}
+		if (i == 0) && (ch == '-' || ch == '+') {
+			continue
+		}
+		// Calculate a number of digits after the decimal point.
+		if ch == '.' {
+			if scale != 0 {
+				return nil, fmt.Errorf("Number contains more than one point")
+			}
+			scale = len(buf) - i - 1
+			continue
+		}
+
+		if ch < '0' || ch > '9' {
+			return nil, fmt.Errorf("Failed to convert symbol '%c' to a digit", ch)
+		}
+		digit := byte(ch - '0')
+		if highNibble {
+			// Add a digit to a high nibble.
+			digit = digit << 4
+			byteBuf = append(byteBuf, digit)
+			highNibble = false
+		} else {
+			if len(byteBuf) == 0 {
+				byteBuf = make([]byte, 1)
+			}
+			// Add a digit to a low nibble.
+			lowByteIdx := len(byteBuf) - 1
+			byteBuf[lowByteIdx] = byteBuf[lowByteIdx] | digit
+			highNibble = true
+		}
+	}
+	if highNibble {
+		// Put a sign to a high nibble.
+		byteBuf = append(byteBuf, signByte)
+	} else {
+		// Put a sign to a low nibble.
+		lowByteIdx := len(byteBuf) - 1
+		byteBuf[lowByteIdx] = byteBuf[lowByteIdx] | signByte
+	}
+	byteBuf = append([]byte{byte(scale)}, byteBuf...)
+
+	return byteBuf, nil
+}
+
+// DecodeStringFromBCD converts a BCD Packed Decimal to a string buffer.
+//
+// The BCD packed decimal byte array, together with an associated scale, is
+// converted to a string. The BCD array is assumed full of digits, and must be
+// ended by a 4-bit sign nibble in the least significant four bits of the final
+// byte. The scale is used (negated) as the exponent of the decimal number.
+// Note that zeroes may have a sign and/or a scale.
+func DecodeStringFromBCD(bcdBuf []byte) (string, error) {
+	// Index of a byte with scale.
+	const scaleIdx = 0
+	scale := int(bcdBuf[scaleIdx])
+
+	// Get a BCD buffer without scale.
+	bcdBuf = bcdBuf[scaleIdx+1:]
+	bufLen := len(bcdBuf)
+
+	// Every nibble contains a digit, and the last low nibble contains a
+	// sign.
+	ndigits := bufLen*2 - 1
+
+	// The first nibble contains 0 if the decimal number has an even number of
+	// digits. Decrease a number of digits if so.
+	if bcdBuf[0]&0xf0 == 0 {
+		ndigits -= 1
+	}
+
+	// Reserve bytes for dot and sign.
+	numLen := ndigits + 2
+	// Reserve bytes for zeroes.
+	if scale >= ndigits {
+		numLen += scale - ndigits
+	}
+
+	var bld strings.Builder
+	bld.Grow(numLen)
+
+	// Add a sign, it is encoded in a low nibble of a last byte.
+	lastByte := bcdBuf[bufLen-1]
+	sign := lastByte & 0x0f
+	if isNegative[sign] {
+		bld.WriteByte('-')
+	}
+
+	// Add missing zeroes to the left side when scale is bigger than a
+	// number of digits and a single missed zero to the right side when
+	// equal.
+	if scale > ndigits {
+		bld.WriteByte('0')
+		bld.WriteByte('.')
+		for diff := scale - ndigits; diff > 0; diff-- {
+			bld.WriteByte('0')
+		}
+	} else if scale == ndigits {
+		bld.WriteByte('0')
+	}
+
+	const MaxDigit = 0x09
+	// Builds a buffer with symbols of decimal number (digits, dot and sign).
+	processNibble := func(nibble byte) {
+		if nibble <= MaxDigit {
+			if ndigits == scale {
+				bld.WriteByte('.')
+			}
+			bld.WriteByte(nibble + '0')
+			ndigits--
+		}
+	}
+
+	for i, bcdByte := range bcdBuf {
+		highNibble := bcdByte >> 4
+		lowNibble := bcdByte & 0x0f
+		// Skip a first high nibble as no digit there.
+		if i != 0 || highNibble != 0 {
+			processNibble(highNibble)
+		}
+		processNibble(lowNibble)
+	}
+
+	return bld.String(), nil
+}

decimal/config.lua

@@ -0,0 +1,41 @@
+local decimal = require('decimal')
+local msgpack = require('msgpack')
+
+-- Do not set listen for now so connector won't be
+-- able to send requests until everything is configured.
+box.cfg{
+    work_dir = os.getenv("TEST_TNT_WORK_DIR"),
+}
+
+box.schema.user.create('test', { password = 'test' , if_not_exists = true })
+box.schema.user.grant('test', 'execute', 'universe', nil, { if_not_exists = true })
+
+local decimal_msgpack_supported = pcall(msgpack.encode, decimal.new(1))
+if not decimal_msgpack_supported then
+    error('Decimal unsupported, use Tarantool 2.2 or newer')
+end
+
+local s = box.schema.space.create('testDecimal', {
+    id = 524,
+    if_not_exists = true,
+})
+s:create_index('primary', {
+    type = 'TREE',
+    parts = {
+        {
+            field = 1,
+            type = 'decimal',
+        },
+    },
+    if_not_exists = true
+})
+s:truncate()
+
+box.schema.user.grant('test', 'read,write', 'space', 'testDecimal', { if_not_exists = true })
+
+-- Set listen only when every other thing is configured.
+box.cfg{
+    listen = os.getenv("TEST_TNT_LISTEN"),
+}
+
+require('console').start()