import BN from "bn.js"

function VarIntFunc(signed: boolean) {
    const encodingLength = (n: number) => {
        if (signed) n *= 2
        if (n < 0) {
            throw Error("varint value is out of bounds")
        }
        const bits = Math.log2(n + 1)
        return Math.ceil(bits / 7) || 1
    }

    const encode = (n: number, buffer?: Buffer | any, offset?: number) => {
        if (n < 0) {
            throw Error("varint value is out of bounds")
        }

        buffer = buffer || Buffer.alloc(encodingLength(n))
        offset = offset || 0
        const nStr = n.toString()
        let bn = new BN(nStr, 10)
        const num255 = new BN(0xff)
        const num128 = new BN(0x80)

        // amino signed varint is multiplied by 2
        if (signed) {
            bn = bn.muln(2)
        }

        let i = 0
        while (bn.gten(0x80)) {
            buffer[offset + i] = bn.and(num255).or(num128).toNumber()
            bn = bn.shrn(7)
            i++
        }

        buffer[offset + i] = bn.andln(0xff)

        // TODO
        // encode.bytes = i + 1

        return buffer
    }

    /**
     * https://github.com/golang/go/blob/master/src/encoding/binary/varint.go#L60
     */
    const decode = (bytes: Buffer | any) => {
        let x = 0
        let s = 0
        for (let i = 0, len = bytes.length; i < len; i++) {
            const b = bytes[i]
            if (b < 0x80) {
                if (i > 9 || (i === 9 && b > 1)) {
                    return 0
                }
                return x | (b << s)
            }
            x |= (b & 0x7f) << s
            s += 7
        }

        return 0
    }

    return { encode, decode, encodingLength }
}

export const UVarInt = VarIntFunc(false)
export const VarInt = VarIntFunc(true)