import {
    assertIsFixedSize,
    Codec,
    combineCodec,
    containsBytes,
    Decoder,
    Encoder,
    fixDecoderSize,
    FixedSizeCodec,
    FixedSizeDecoder,
    FixedSizeEncoder,
    fixEncoderSize,
    ReadonlyUint8Array,
    transformDecoder,
    transformEncoder,
    VariableSizeCodec,
    VariableSizeDecoder,
    VariableSizeEncoder,
} from '@solana/codecs-core';
import {
    FixedSizeNumberCodec,
    FixedSizeNumberDecoder,
    FixedSizeNumberEncoder,
    getU8Decoder,
    getU8Encoder,
    NumberCodec,
    NumberDecoder,
    NumberEncoder,
} from '@solana/codecs-numbers';

import { getBooleanDecoder, getBooleanEncoder } from './boolean';
import { getConstantDecoder, getConstantEncoder } from './constant';
import { getTupleDecoder, getTupleEncoder } from './tuple';
import { getUnionDecoder, getUnionEncoder } from './union';
import { getUnitDecoder, getUnitEncoder } from './unit';

/**
 * Defines the configuration options for nullable codecs.
 *
 * This configuration controls how nullable values are encoded and decoded.
 *
 * By default, nullable values are prefixed with a `u8` (0 = `null`, 1 = present).
 * The `noneValue` and `prefix` options allow customizing this behavior.
 *
 * @typeParam TPrefix - A number codec, encoder, or decoder used as the presence prefix.
 *
 * @see {@link getNullableEncoder}
 * @see {@link getNullableDecoder}
 * @see {@link getNullableCodec}
 */
export type NullableCodecConfig<TPrefix extends NumberCodec | NumberDecoder | NumberEncoder> = {
    /**
     * Specifies how `null` values are represented in the encoded data.
     *
     * - By default, `null` values are omitted from encoding.
     * - `'zeroes'`: The bytes allocated for the value are filled with zeroes. This requires a fixed-size codec.
     * - Custom byte array: `null` values are replaced with a predefined byte sequence. This results in a variable-size codec.
     *
     * @defaultValue No explicit `noneValue` is used; `null` values are omitted.
     */
    noneValue?: ReadonlyUint8Array | 'zeroes';

    /**
     * The presence prefix used to distinguish between `null` and present values.
     *
     * - By default, a `u8` prefix is used (`0 = null`, `1 = present`).
     * - Custom number codec: Allows defining a different number size for the prefix.
     * - `null`: No prefix is used; `noneValue` (if provided) determines `null`.
     *   If no `noneValue` is set, `null` is identified by the absence of bytes.
     *
     * @defaultValue `u8` prefix.
     */
    prefix?: TPrefix | null;
};

/**
 * Returns an encoder for optional values, allowing `null` values to be encoded.
 *
 * This encoder serializes an optional value using a configurable approach:
 * - By default, a `u8` prefix is used (0 = `null`, 1 = present). This can be customized or disabled.
 * - If `noneValue: 'zeroes'` is set, `null` values are encoded as zeroes.
 * - If `noneValue` is a byte array, `null` values are replaced with the provided constant.
 *
 * For more details, see {@link getNullableCodec}.
 *
 * @typeParam TFrom - The type of the main value being encoded.
 *
 * @param item - The encoder for the value that may be present.
 * @param config - Configuration options for encoding optional values.
 * @returns A `FixedSizeEncoder` or `VariableSizeEncoder` for encoding nullable values.
 *
 * @example
 * Encoding an optional number.
 * ```ts
 * const encoder = getNullableEncoder(getU32Encoder());
 *
 * encoder.encode(null); // 0x00
 * encoder.encode(42);   // 0x012a000000
 * ```
 *
 * @see {@link getNullableCodec}
 */
export function getNullableEncoder<TFrom, TSize extends number>(
    item: FixedSizeEncoder<TFrom, TSize>,
    config: NullableCodecConfig<NumberEncoder> & { noneValue: 'zeroes'; prefix: null },
): FixedSizeEncoder<TFrom | null, TSize>;
export function getNullableEncoder<TFrom>(
    item: FixedSizeEncoder<TFrom>,
    config: NullableCodecConfig<FixedSizeNumberEncoder> & { noneValue: 'zeroes' },
): FixedSizeEncoder<TFrom | null>;
export function getNullableEncoder<TFrom>(
    item: FixedSizeEncoder<TFrom>,
    config: NullableCodecConfig<NumberEncoder> & { noneValue: 'zeroes' },
): VariableSizeEncoder<TFrom | null>;
export function getNullableEncoder<TFrom>(
    item: Encoder<TFrom>,
    config?: NullableCodecConfig<NumberEncoder> & { noneValue?: ReadonlyUint8Array },
): VariableSizeEncoder<TFrom | null>;
export function getNullableEncoder<TFrom>(
    item: Encoder<TFrom>,
    config: NullableCodecConfig<NumberEncoder> = {},
): Encoder<TFrom | null> {
    const prefix = (() => {
        if (config.prefix === null) {
            return transformEncoder(getUnitEncoder(), (_boolean: boolean) => undefined);
        }
        return getBooleanEncoder({ size: config.prefix ?? getU8Encoder() });
    })();
    const noneValue = (() => {
        if (config.noneValue === 'zeroes') {
            assertIsFixedSize(item);
            return fixEncoderSize(getUnitEncoder(), item.fixedSize);
        }
        if (!config.noneValue) {
            return getUnitEncoder();
        }
        return getConstantEncoder(config.noneValue);
    })();

    return getUnionEncoder(
        [
            transformEncoder(getTupleEncoder([prefix, noneValue]), (_value: null): [boolean, void] => [
                false,
                undefined,
            ]),
            transformEncoder(getTupleEncoder([prefix, item]), (value: TFrom): [boolean, TFrom] => [true, value]),
        ],
        variant => Number(variant !== null),
    );
}

/**
 * Returns a decoder for optional values, allowing `null` values to be recognized.
 *
 * This decoder deserializes an optional value using a configurable approach:
 * - By default, a `u8` prefix is used (0 = `null`, 1 = present). This can be customized or disabled.
 * - If `noneValue: 'zeroes'` is set, `null` values are identified by zeroes.
 * - If `noneValue` is a byte array, `null` values match the provided constant.
 *
 * For more details, see {@link getNullableCodec}.
 *
 * @typeParam TTo - The type of the main value being decoded.
 *
 * @param item - The decoder for the value that may be present.
 * @param config - Configuration options for decoding optional values.
 * @returns A `FixedSizeDecoder` or `VariableSizeDecoder` for decoding nullable values.
 *
 * @example
 * Decoding an optional number.
 * ```ts
 * const decoder = getNullableDecoder(getU32Decoder());
 *
 * decoder.decode(new Uint8Array([0x00])); // null
 * decoder.decode(new Uint8Array([0x01, 0x2a, 0x00, 0x00, 0x00])); // 42
 * ```
 *
 * @see {@link getNullableCodec}
 */
export function getNullableDecoder<TTo, TSize extends number>(
    item: FixedSizeDecoder<TTo, TSize>,
    config: NullableCodecConfig<NumberDecoder> & { noneValue: 'zeroes'; prefix: null },
): FixedSizeDecoder<TTo | null, TSize>;
export function getNullableDecoder<TTo>(
    item: FixedSizeDecoder<TTo>,
    config: NullableCodecConfig<FixedSizeNumberDecoder> & { noneValue: 'zeroes' },
): FixedSizeDecoder<TTo | null>;
export function getNullableDecoder<TTo>(
    item: FixedSizeDecoder<TTo>,
    config: NullableCodecConfig<NumberDecoder> & { noneValue: 'zeroes' },
): VariableSizeDecoder<TTo | null>;
export function getNullableDecoder<TTo>(
    item: Decoder<TTo>,
    config?: NullableCodecConfig<NumberDecoder> & { noneValue?: ReadonlyUint8Array },
): VariableSizeDecoder<TTo | null>;
export function getNullableDecoder<TTo>(
    item: Decoder<TTo>,
    config: NullableCodecConfig<NumberDecoder> = {},
): Decoder<TTo | null> {
    const prefix = (() => {
        if (config.prefix === null) {
            return transformDecoder(getUnitDecoder(), () => false);
        }
        return getBooleanDecoder({ size: config.prefix ?? getU8Decoder() });
    })();
    const noneValue = (() => {
        if (config.noneValue === 'zeroes') {
            assertIsFixedSize(item);
            return fixDecoderSize(getUnitDecoder(), item.fixedSize);
        }
        if (!config.noneValue) {
            return getUnitDecoder();
        }
        return getConstantDecoder(config.noneValue);
    })();

    return getUnionDecoder(
        [
            transformDecoder(getTupleDecoder([prefix, noneValue]), () => null),
            transformDecoder(getTupleDecoder([prefix, item]), ([, value]): TTo => value),
        ],
        (bytes, offset) => {
            if (config.prefix === null && !config.noneValue) {
                return Number(offset < bytes.length);
            }
            if (config.prefix === null && config.noneValue != null) {
                const zeroValue =
                    config.noneValue === 'zeroes' ? new Uint8Array(noneValue.fixedSize).fill(0) : config.noneValue;
                return containsBytes(bytes, zeroValue, offset) ? 0 : 1;
            }
            return Number(prefix.read(bytes, offset)[0]);
        },
    );
}

/**
 * Returns a codec for encoding and decoding optional values, allowing `null` values to be handled.
 *
 * This codec serializes and deserializes optional values using a configurable approach:
 * - By default, a `u8` prefix is used (0 = `null`, 1 = present).
 *    This can be customized using a custom number codec or even disabled by setting
 *    the `prefix` to `null`.
 * - If `noneValue: 'zeroes'` is set, `null` values are encoded/decoded as zeroes.
 * - If `noneValue` is a byte array, `null` values are represented by the provided constant.
 *
 * For more details on the configuration options, see {@link NullableCodecConfig}.
 *
 * @typeParam TFrom - The type of the main value being encoded.
 * @typeParam TTo - The type of the main value being decoded.
 *
 * @param item - The codec for the value that may be present.
 * @param config - Configuration options for encoding and decoding optional values.
 * @returns A `FixedSizeCodec` or `VariableSizeCodec` for encoding and decoding nullable values.
 *
 * @example
 * Encoding and decoding an optional number using a `u8` prefix (default).
 * ```ts
 * const codec = getNullableCodec(getU32Codec());
 *
 * codec.encode(null); // 0x00
 * codec.encode(42);   // 0x012a000000
 *
 * codec.decode(new Uint8Array([0x00])); // null
 * codec.decode(new Uint8Array([0x01, 0x2a, 0x00, 0x00, 0x00])); // 42
 * ```
 *
 * @example
 * Encoding and decoding an optional number using a fixed-size codec, by filling `null` values with zeroes.
 * ```ts
 * const codec = getNullableCodec(getU32Codec(), { noneValue: 'zeroes' });
 *
 * codec.encode(null); // 0x0000000000
 * codec.encode(42);   // 0x012a000000
 *
 * codec.decode(new Uint8Array([0x00, 0x00, 0x00, 0x00, 0x00])); // null
 * codec.decode(new Uint8Array([0x01, 0x2a, 0x00, 0x00, 0x00])); // 42
 * ```
 *
 * @example
 * Encoding and decoding `null` values with zeroes and no prefix.
 * ```ts
 * const codec = getNullableCodec(getU32Codec(), {
 *   noneValue: 'zeroes',
 *   prefix: null,
 * });
 *
 * codec.encode(null); // 0x00000000
 * codec.encode(42);   // 0x2a000000
 *
 * codec.decode(new Uint8Array([0x00, 0x00, 0x00, 0x00])); // null
 * codec.decode(new Uint8Array([0x2a, 0x00, 0x00, 0x00])); // 42
 * ```
 *
 * @example
 * Encoding and decoding `null` values with a custom byte sequence and no prefix.
 * ```ts
 * const codec = getNullableCodec(getU16Codec(), {
 *   noneValue: new Uint8Array([0xff, 0xff]),
 *   prefix: null,
 * });
 *
 * codec.encode(null); // 0xffff
 * codec.encode(42); // 0x2a00
 *
 * codec.decode(new Uint8Array([0xff, 0xff])); // null
 * codec.decode(new Uint8Array([0x2a, 0x00])); // 42
 * ```
 *
 * @example
 * Identifying `null` values by the absence of bytes.
 * ```ts
 * const codec = getNullableCodec(getU16Codec(), { prefix: null });
 *
 * codec.encode(null); // Empty bytes
 * codec.encode(42); // 0x2a00
 *
 * codec.decode(new Uint8Array([])); // null
 * codec.decode(new Uint8Array([0x2a, 0x00])); // 42
 * ```
 *
 * @remarks
 * Separate {@link getNullableEncoder} and {@link getNullableDecoder} functions are available.
 *
 * ```ts
 * const bytes = getNullableEncoder(getU32Encoder()).encode(42);
 * const value = getNullableDecoder(getU32Decoder()).decode(bytes);
 * ```
 *
 * @see {@link getNullableEncoder}
 * @see {@link getNullableDecoder}
 */
export function getNullableCodec<TFrom, TTo extends TFrom, TSize extends number>(
    item: FixedSizeCodec<TFrom, TTo, TSize>,
    config: NullableCodecConfig<NumberCodec> & { noneValue: 'zeroes'; prefix: null },
): FixedSizeCodec<TFrom | null, TTo | null, TSize>;
export function getNullableCodec<TFrom, TTo extends TFrom = TFrom>(
    item: FixedSizeCodec<TFrom, TTo>,
    config: NullableCodecConfig<FixedSizeNumberCodec> & { noneValue: 'zeroes' },
): FixedSizeCodec<TFrom | null, TTo | null>;
export function getNullableCodec<TFrom, TTo extends TFrom = TFrom>(
    item: FixedSizeCodec<TFrom, TTo>,
    config: NullableCodecConfig<NumberCodec> & { noneValue: 'zeroes' },
): VariableSizeCodec<TFrom | null, TTo | null>;
export function getNullableCodec<TFrom, TTo extends TFrom = TFrom>(
    item: Codec<TFrom, TTo>,
    config?: NullableCodecConfig<NumberCodec> & { noneValue?: ReadonlyUint8Array },
): VariableSizeCodec<TFrom | null, TTo | null>;
export function getNullableCodec<TFrom, TTo extends TFrom = TFrom>(
    item: Codec<TFrom, TTo>,
    config: NullableCodecConfig<NumberCodec> = {},
): Codec<TFrom | null, TTo | null> {
    type ConfigCast = NullableCodecConfig<NumberCodec> & { noneValue?: ReadonlyUint8Array };
    return combineCodec(
        getNullableEncoder<TFrom>(item, config as ConfigCast),
        getNullableDecoder<TTo>(item, config as ConfigCast),
    );
}