// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

using System.Formats.Asn1;

namespace Microsoft.DotNet.Build.Tasks.FileCatalog;

/// <summary>
/// DER-encodes the per-member ASN.1 structures inside a Microsoft Catalog (.cat) file,
/// matching the layout produced by <c>makecat.exe</c> for a V2 (SHA-256) catalog.
///
/// For each cataloged file, makecat emits two <c>TrustedSubject</c> members:
/// one keyed by the raw SHA-1 hash and one keyed by the raw SHA-256 hash. Both carry a
/// <c>CAT_MEMBERINFO2</c> marker attribute; the SHA-256 member additionally carries an
/// <c>SPC_INDIRECT_DATA</c> attribute holding the SHA-256 digest.
/// </summary>
internal static class CatalogMemberEncoder
{
    /// <summary>
    /// Writes a single <c>TrustedSubject</c> SEQUENCE.
    /// </summary>
    /// <param name="writer">The DER writer.</param>
    /// <param name="identifier">The raw hash bytes that identify the member.</param>
    /// <param name="sha256Digest">
    /// When non-null, the member is a SHA-256 member and also carries an
    /// <c>SPC_INDIRECT_DATA</c> attribute with this digest (equal to <paramref name="identifier"/>).
    /// When null, the member is a SHA-1 member carrying only <c>CAT_MEMBERINFO2</c>.
    /// </param>
    public static void WriteTrustedSubject(AsnWriter writer, byte[] identifier, byte[]? sha256Digest)
    {
        writer.PushSequence();

        // subjectIdentifier: OCTET STRING of the raw hash bytes.
        writer.WriteOctetString(identifier);

        // subjectAttributes: SET OF Attribute (DER-sorted by PushSetOf).
        writer.PushSetOf();

        WriteCatMemberInfo2Attribute(writer);
        if (sha256Digest is not null)
        {
            WriteSpcIndirectDataAttribute(writer, sha256Digest);
        }

        writer.PopSetOf();
        writer.PopSequence();
    }

    /// <summary>
    /// Writes the CAT_MEMBERINFO2 attribute:
    ///   Attribute { OID 1.3.6.1.4.1.311.12.2.3, SET { [2] &lt;empty&gt; } }
    /// (encoded bytes: 30 10 06 0a 2b 06 01 04 01 82 37 0c 02 03 31 02 82 00)
    /// </summary>
    private static void WriteCatMemberInfo2Attribute(AsnWriter writer)
    {
        writer.PushSequence();
        writer.WriteObjectIdentifier(CatalogOids.CatMemberInfo2);
        writer.PushSetOf();
        // A single context-[2] element with empty content ("82 00").
        writer.WriteNull(new Asn1Tag(TagClass.ContextSpecific, tagValue: 2));
        writer.PopSetOf();
        writer.PopSequence();
    }

    /// <summary>
    /// Writes the SPC_INDIRECT_DATA attribute carrying the SHA-256 file hash:
    ///   Attribute { OID 1.3.6.1.4.1.311.2.1.4,
    ///               SET { SEQUENCE {
    ///                 data   SEQUENCE { OID 1.3.6.1.4.1.311.2.1.25, [2] { [0] &lt;empty&gt; } },
    ///                 digest SEQUENCE { SEQUENCE { OID sha256, NULL }, OCTET STRING &lt;hash&gt; } } } }
    /// </summary>
    private static void WriteSpcIndirectDataAttribute(AsnWriter writer, byte[] sha256Digest)
    {
        writer.PushSequence();
        writer.WriteObjectIdentifier(CatalogOids.SpcIndirectData);
        writer.PushSetOf();

        // SpcIndirectDataContent
        writer.PushSequence();

        // data: SpcAttributeTypeAndOptionalValue { type, value }
        writer.PushSequence();
        writer.WriteObjectIdentifier(CatalogOids.SpcLinkData);
        // value: file [2] EXPLICIT { unicode [0] IMPLICIT BMPString "" }  ->  a2 02 80 00
        Asn1Tag fileTag = new(TagClass.ContextSpecific, tagValue: 2, isConstructed: true);
        writer.PushSequence(fileTag);
        Asn1Tag unicodeTag = new(TagClass.ContextSpecific, tagValue: 0, isConstructed: false);
        writer.WriteCharacterString(UniversalTagNumber.BMPString, string.Empty, unicodeTag);
        writer.PopSequence(fileTag);
        writer.PopSequence();

        // messageDigest: DigestInfo { AlgorithmIdentifier { OID sha256, NULL }, OCTET STRING digest }
        writer.PushSequence();
        writer.PushSequence();
        writer.WriteObjectIdentifier(CatalogOids.Sha256);
        writer.WriteNull();
        writer.PopSequence();
        writer.WriteOctetString(sha256Digest);
        writer.PopSequence();

        writer.PopSequence(); // SpcIndirectDataContent

        writer.PopSetOf();
        writer.PopSequence();
    }
}