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- #region Using
- #endregion
- namespace SharpCompress.Compressors.PPMd.I1
- {
- /// <summary>
- /// SEE2 (secondary escape estimation) contexts for PPM contexts with masked symbols.
- /// </summary>
- /// <remarks>
- /// <para>
- /// This must be a class rather than a structure because MakeEscapeFrequency returns a See2Context
- /// instance from the see2Contexts array. The caller (for example, EncodeSymbol2) then updates the
- /// returned See2Context instance and expects the updates to be reflected in the see2Contexts array.
- /// This would not happen if this were a structure.
- /// </para>
- /// <remarks>
- /// Note that in most cases fields are used rather than properties for performance reasons (for example,
- /// <see cref="_shift"/> is a field rather than a property).
- /// </remarks>
- /// </remarks>
- internal class See2Context
- {
- private const byte PERIOD_BIT_COUNT = 7;
- public ushort _summary;
- public byte _shift;
- public byte _count;
- public void Initialize(uint initialValue)
- {
- _shift = PERIOD_BIT_COUNT - 4;
- _summary = (ushort)(initialValue << _shift);
- _count = 7;
- }
- public uint Mean()
- {
- uint value = (uint)(_summary >> _shift);
- _summary = (ushort)(_summary - value);
- return (uint)(value + ((value == 0) ? 1 : 0));
- }
- public void Update()
- {
- if (_shift < PERIOD_BIT_COUNT && --_count == 0)
- {
- _summary += _summary;
- _count = (byte)(3 << _shift++);
- }
- }
- }
- }
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