// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

using System;
using System.Diagnostics.Contracts;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
using Microsoft.Toolkit.HighPerformance.Enumerables;
using Microsoft.Toolkit.HighPerformance.Helpers.Internals;

namespace Microsoft.Toolkit.HighPerformance.Extensions
{
    /// <summary>
    /// Helpers for working with the <see cref="string"/> type.
    /// </summary>
    public static class StringExtensions
    {
        /// <summary>
        /// Returns a reference to the first element within a given <see cref="string"/>, with no bounds checks.
        /// </summary>
        /// <param name="text">The input <see cref="string"/> instance.</param>
        /// <returns>A reference to the first element within <paramref name="text"/>, or the location it would have used, if <paramref name="text"/> is empty.</returns>
        /// <remarks>This method doesn't do any bounds checks, therefore it is responsibility of the caller to perform checks in case the returned value is dereferenced.</remarks>
        [Pure]
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static ref char DangerousGetReference(this string text)
        {
#if NETCOREAPP3_1
            return ref Unsafe.AsRef(text.GetPinnableReference());
#elif NETCOREAPP2_1
            var stringData = Unsafe.As<RawStringData>(text);

            return ref stringData.Data;
#else
            return ref MemoryMarshal.GetReference(text.AsSpan());
#endif
        }

        /// <summary>
        /// Returns a reference to an element at a specified index within a given <see cref="string"/>, with no bounds checks.
        /// </summary>
        /// <param name="text">The input <see cref="string"/> instance.</param>
        /// <param name="i">The index of the element to retrieve within <paramref name="text"/>.</param>
        /// <returns>A reference to the element within <paramref name="text"/> at the index specified by <paramref name="i"/>.</returns>
        /// <remarks>This method doesn't do any bounds checks, therefore it is responsibility of the caller to ensure the <paramref name="i"/> parameter is valid.</remarks>
        [Pure]
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static ref char DangerousGetReferenceAt(this string text, int i)
        {
#if NETCOREAPP3_1
            ref char r0 = ref Unsafe.AsRef(text.GetPinnableReference());
#elif NETCOREAPP2_1
            ref char r0 = ref Unsafe.As<RawStringData>(text).Data;
#else
            ref char r0 = ref MemoryMarshal.GetReference(text.AsSpan());
#endif
            ref char ri = ref Unsafe.Add(ref r0, i);

            return ref ri;
        }

#if NETCOREAPP2_1
        // Description adapted from CoreCLR: see https://source.dot.net/#System.Private.CoreLib/src/System/Runtime/CompilerServices/RuntimeHelpers.CoreCLR.cs,285.
        // CLR strings are laid out in memory as follows:
        // [ sync block || pMethodTable || length || string data .. ]
        //                 ^                         ^
        //                 |                         \-- ref Unsafe.As<RawStringData>(text).Data
        //                 \-- string
        // The reference to RawStringData.Data points to the first character in the
        // string, skipping over the sync block, method table and string length.
        [StructLayout(LayoutKind.Explicit)]
        private sealed class RawStringData
        {
#pragma warning disable CS0649 // Unassigned fields
#pragma warning disable SA1401 // Fields should be private
            [FieldOffset(4)]
            public char Data;
#pragma warning restore CS0649
#pragma warning restore SA1401
        }
#endif

        /// <summary>
        /// Counts the number of occurrences of a given character into a target <see cref="string"/> instance.
        /// </summary>
        /// <param name="text">The input <see cref="string"/> instance to read.</param>
        /// <param name="c">The character to look for.</param>
        /// <returns>The number of occurrences of <paramref name="c"/> in <paramref name="text"/>.</returns>
        [Pure]
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static int Count(this string text, char c)
        {
            ref char r0 = ref text.DangerousGetReference();
            IntPtr length = (IntPtr)text.Length;

            return SpanHelper.Count(ref r0, length, c);
        }

        /// <summary>
        /// Enumerates the items in the input <see cref="string"/> instance, as pairs of value/index values.
        /// This extension should be used directly within a <see langword="foreach"/> loop:
        /// <code>
        /// string text = "Hello, world!";
        ///
        /// foreach (var item in text.Enumerate())
        /// {
        ///     // Access the index and value of each item here...
        ///     int index = item.Index;
        ///     string value = item.Value;
        /// }
        /// </code>
        /// The compiler will take care of properly setting up the <see langword="foreach"/> loop with the type returned from this method.
        /// </summary>
        /// <param name="text">The source <see cref="string"/> to enumerate.</param>
        /// <returns>A wrapper type that will handle the value/index enumeration for <paramref name="text"/>.</returns>
        /// <remarks>The returned <see cref="ReadOnlySpanEnumerable{T}"/> value shouldn't be used directly: use this extension in a <see langword="foreach"/> loop.</remarks>
        [Pure]
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static ReadOnlySpanEnumerable<char> Enumerate(this string text)
        {
            return new ReadOnlySpanEnumerable<char>(text.AsSpan());
        }

        /// <summary>
        /// Tokenizes the values in the input <see cref="string"/> instance using a specified separator.
        /// This extension should be used directly within a <see langword="foreach"/> loop:
        /// <code>
        /// string text = "Hello, world!";
        ///
        /// foreach (var token in text.Tokenize(','))
        /// {
        ///     // Access the tokens here...
        /// }
        /// </code>
        /// The compiler will take care of properly setting up the <see langword="foreach"/> loop with the type returned from this method.
        /// </summary>
        /// <param name="text">The source <see cref="string"/> to tokenize.</param>
        /// <param name="separator">The separator character to use.</param>
        /// <returns>A wrapper type that will handle the tokenization for <paramref name="text"/>.</returns>
        /// <remarks>The returned <see cref="ReadOnlySpanTokenizer{T}"/> value shouldn't be used directly: use this extension in a <see langword="foreach"/> loop.</remarks>
        [Pure]
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static ReadOnlySpanTokenizer<char> Tokenize(this string text, char separator)
        {
            return new ReadOnlySpanTokenizer<char>(text.AsSpan(), separator);
        }

        /// <summary>
        /// Gets a content hash from the input <see cref="string"/> instance using the Djb2 algorithm.
        /// For more info, see the documentation for <see cref="ReadOnlySpanExtensions.GetDjb2HashCode{T}"/>.
        /// </summary>
        /// <param name="text">The source <see cref="string"/> to enumerate.</param>
        /// <returns>The Djb2 value for the input <see cref="string"/> instance.</returns>
        /// <remarks>The Djb2 hash is fully deterministic and with no random components.</remarks>
        [Pure]
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static int GetDjb2HashCode(this string text)
        {
            ref char r0 = ref text.DangerousGetReference();
            IntPtr length = (IntPtr)text.Length;

            return SpanHelper.GetDjb2HashCode(ref r0, length);
        }
    }
}