// 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 Microsoft.CodeAnalysis.Text;
 
namespace Microsoft.CodeAnalysis.FileHeaders;
 
/// <summary>
/// Contains the parsed file header information for a syntax tree.
/// </summary>
internal readonly struct FileHeader
{
    /// <summary>
    /// The location in the source where the file header was expected to start.
    /// </summary>
    private readonly int _fileHeaderStart;
 
    /// <summary>
    /// The length of the prefix indicating the start of a comment. For example:
    /// <list type="bullet">
    ///   <item>
    ///     <term>C#</term>
    ///     <description>2, for the length of <c>//</c>.</description>
    ///   </item>
    ///   <item>
    ///     <term>Visual Basic</term>
    ///     <description>1, for the length of <c>'</c>.</description>
    ///   </item>
    /// </list>
    /// </summary>
    private readonly int _commentPrefixLength;
 
    /// <summary>
    /// Initializes a new instance of the <see cref="FileHeader"/> struct.
    /// </summary>
    /// <param name="copyrightText">The copyright string, as parsed from the header.</param>
    /// <param name="fileHeaderStart">The offset within the file at which the header started.</param>
    /// <param name="fileHeaderEnd">The offset within the file at which the header ended.</param>
    internal FileHeader(string copyrightText, int fileHeaderStart, int fileHeaderEnd, int commentPrefixLength)
        : this(fileHeaderStart, isMissing: false)
    {
        // Currently unused
        _ = fileHeaderEnd;
 
        CopyrightText = copyrightText;
        _commentPrefixLength = commentPrefixLength;
    }
 
    /// <summary>
    /// Initializes a new instance of the <see cref="FileHeader"/> struct.
    /// </summary>
    /// <param name="fileHeaderStart">The offset within the file at which the header started, or was expected to start.</param>
    /// <param name="isMissing"><see langword="true"/> if the file header is missing; otherwise, <see langword="false"/>.</param>
    private FileHeader(int fileHeaderStart, bool isMissing)
    {
        _fileHeaderStart = fileHeaderStart;
        _commentPrefixLength = 0;
 
        IsMissing = isMissing;
        CopyrightText = "";
    }
 
    /// <summary>
    /// Gets a value indicating whether the file header is missing.
    /// </summary>
    /// <value>
    /// <see langword="true"/> if the file header is missing; otherwise, <see langword="false"/>.
    /// </value>
    internal bool IsMissing { get; }
 
    /// <summary>
    /// Gets the copyright text, as parsed from the header.
    /// </summary>
    /// <value>
    /// The copyright text, as parsed from the header.
    /// </value>
    internal string CopyrightText { get; }
 
    /// <summary>
    /// Gets a <see cref="FileHeader"/> instance representing a missing file header starting at the specified
    /// position.
    /// </summary>
    /// <param name="fileHeaderStart">The location at which a file header was expected. This will typically be the
    /// start of the first line after any directive trivia (<see cref="SyntaxTrivia.IsDirective"/>) to account for
    /// source suppressions.</param>
    /// <returns>
    /// A <see cref="FileHeader"/> instance representing a missing file header.
    /// </returns>
    internal static FileHeader MissingFileHeader(int fileHeaderStart)
        => new(fileHeaderStart, isMissing: true);
 
    /// <summary>
    /// Gets the location representing the start of the file header.
    /// </summary>
    /// <param name="syntaxTree">The syntax tree to use for generating the location.</param>
    /// <returns>The location representing the start of the file header.</returns>
    internal Location GetLocation(SyntaxTree syntaxTree)
    {
        if (IsMissing)
        {
            return Location.Create(syntaxTree, new TextSpan(_fileHeaderStart, 0));
        }
 
        return Location.Create(syntaxTree, new TextSpan(_fileHeaderStart, _commentPrefixLength));
    }
}