Joseph Doherty
241
src/ZB.MOM.WW.OtOpcUa.Driver.S7/S7BlockCoalescingPlanner.cs
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241
src/ZB.MOM.WW.OtOpcUa.Driver.S7/S7BlockCoalescingPlanner.cs
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namespace ZB.MOM.WW.OtOpcUa.Driver.S7;
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/// <summary>
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/// Block-read coalescing planner for the S7 driver (PR-S7-B2). Where the
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/// <see cref="S7ReadPacker"/> coalesces N scalar tags into ⌈N/19⌉
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/// <c>Plc.ReadMultipleVarsAsync</c> PDUs, this planner takes one further pass:
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/// it groups same-area, same-DB tags by contiguous byte range and folds them
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/// into a single <c>Plc.ReadBytesAsync</c> covering the merged span. The
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/// response is sliced client-side per tag so the per-tag decode path is
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/// unchanged.
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/// </summary>
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/// <remarks>
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/// <para>
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/// <b>Why coalesce</b>: Reading <c>DB1.DBW0</c> + <c>DB1.DBW2</c> +
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/// <c>DB1.DBW4</c> as three multi-var items still uses three slots in a
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/// single PDU; coalescing into one 6-byte byte-range read drops the per-item
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/// framing entirely and makes the request fit in fewer (sometimes zero
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/// additional) PDUs. On a typical contiguous DB the wire-level reduction is
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/// 50:1 for 50 contiguous DBWs.
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/// </para>
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/// <para>
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/// <b>Gap-merge threshold</b>: The planner merges adjacent tag ranges when
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/// the gap between them is at most the <c>gapMergeBytes</c> argument to
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/// <see cref="Plan"/>. The default <see cref="DefaultGapMergeBytes"/> is
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/// 16 bytes — over-fetching 16 bytes is cheaper than one extra PDU
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/// (240-byte default PDU envelope, ~18 bytes per request frame). Operators
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/// can tune the threshold per driver instance via
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/// <see cref="S7DriverOptions.BlockCoalescingGapBytes"/>.
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/// </para>
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/// <para>
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/// <b>Opaque-size opt-out</b>: STRING / WSTRING / CHAR / WCHAR and DTL /
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/// DT / S5TIME / TIME / TOD / DATE-as-DateTime tags carry a header (or
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/// have a per-tag width that varies with <c>StringLength</c>) and are
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/// flagged <c>OpaqueSize=true</c>. The planner emits these as standalone
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/// single-tag ranges and never merges them into a sibling block — the
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/// per-tag decode path needs an exact byte slice and a wrong slice from
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/// a coalesced read would silently corrupt every neighbour.
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/// </para>
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/// <para>
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/// <b>Order-preserving</b>: Each <see cref="BlockReadRange"/> carries a list
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/// of <see cref="TagSlice"/> values pointing back at the original
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/// caller-index. The driver's <c>ReadAsync</c> uses the index to write the
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/// decoded value into the correct slot of the result array, so caller
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/// ordering of the input <c>fullReferences</c> is preserved across the
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/// coalescing step.
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/// </para>
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/// </remarks>
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internal static class S7BlockCoalescingPlanner
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{
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/// <summary>Default gap-merge threshold in bytes.</summary>
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internal const int DefaultGapMergeBytes = 16;
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/// <summary>
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/// One coalesced byte-range request. The driver issues a single
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/// <c>Plc.ReadBytesAsync</c> covering <see cref="StartByte"/>..
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/// <see cref="StartByte"/>+<see cref="ByteCount"/>; each entry in
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/// <see cref="Tags"/> carries the offset within the response buffer to
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/// slice for that tag.
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/// </summary>
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internal sealed record BlockReadRange(
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S7Area Area,
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int DbNumber,
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int StartByte,
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int ByteCount,
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IReadOnlyList<TagSlice> Tags);
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/// <summary>
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/// One tag's slot inside a <see cref="BlockReadRange"/>. <see cref="OffsetInBlock"/>
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/// is the byte offset within the coalesced buffer; <see cref="ByteCount"/> is the
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/// per-tag width that the slice covers.
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/// </summary>
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/// <param name="CallerIndex">Original index in the caller's <c>fullReferences</c> list.</param>
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/// <param name="OffsetInBlock">Byte offset into <see cref="BlockReadRange"/>'s buffer.</param>
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/// <param name="ByteCount">Bytes the tag claims from the buffer.</param>
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internal sealed record TagSlice(int CallerIndex, int OffsetInBlock, int ByteCount);
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/// <summary>
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/// Input row. Captures everything the planner needs to make a coalescing
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/// decision without needing the full <see cref="S7TagDefinition"/> graph.
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/// </summary>
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/// <param name="CallerIndex">Caller-supplied stable index used to thread the decoded value back.</param>
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/// <param name="Area">Memory area; M and DB never merge into the same range.</param>
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/// <param name="DbNumber">DB number when <see cref="Area"/> is DataBlock; 0 otherwise.</param>
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/// <param name="StartByte">Byte offset in the area where the tag's storage begins.</param>
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/// <param name="ByteCount">On-wire byte width of the tag.</param>
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/// <param name="OpaqueSize">
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/// True for tags whose effective decode width is variable / header-prefixed
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/// (STRING/WSTRING/CHAR/WCHAR and structured timestamps DTL/DT/etc.) so the
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/// planner skips them — they emit standalone reads and never merge with
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/// neighbours.
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/// </param>
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internal sealed record TagSpec(
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int CallerIndex,
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S7Area Area,
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int DbNumber,
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int StartByte,
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int ByteCount,
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bool OpaqueSize);
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/// <summary>
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/// Plan a list of byte-range reads from <paramref name="tags"/>. Same-area /
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/// same-DB rows are sorted by <see cref="TagSpec.StartByte"/> then merged
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/// greedily when the gap between their byte ranges is <=
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/// <paramref name="gapMergeBytes"/>. Opaque-size rows always emit as their
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/// own single-tag range and never extend a sibling block.
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/// </summary>
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/// <remarks>
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/// Order of returned ranges is not significant — the driver issues them
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/// sequentially against the same connection gate so wire-level ordering is
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/// determined by the loop, not by this list. The planner DOES preserve
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/// the caller-index inside each range so the per-tag decode result lands
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/// in the correct slot of the response array.
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/// </remarks>
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internal static List<BlockReadRange> Plan(IReadOnlyList<TagSpec> tags, int gapMergeBytes = DefaultGapMergeBytes)
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{
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if (gapMergeBytes < 0)
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throw new ArgumentOutOfRangeException(nameof(gapMergeBytes), "Gap-merge threshold must be non-negative.");
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var ranges = new List<BlockReadRange>(tags.Count);
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if (tags.Count == 0) return ranges;
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// Phase 1: opaque rows emit as standalone single-tag ranges. Strip them
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// out of the merge candidate set so neighbour ranges don't accidentally
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// straddle a STRING header / DTL block.
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var mergeable = new List<TagSpec>(tags.Count);
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foreach (var t in tags)
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{
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if (t.OpaqueSize)
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{
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ranges.Add(new BlockReadRange(
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t.Area, t.DbNumber, t.StartByte, t.ByteCount,
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[new TagSlice(t.CallerIndex, OffsetInBlock: 0, t.ByteCount)]));
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}
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else
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{
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mergeable.Add(t);
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}
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}
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// Phase 2: bucket by (Area, DbNumber). Memory M and DataBlock DB1 (etc.)
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// share neither the wire request type nor an addressable space, so they
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// can never coalesce.
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var groups = mergeable.GroupBy(t => (t.Area, t.DbNumber));
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foreach (var group in groups)
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{
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// Sort ascending by start byte so the greedy merge below is O(n).
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// Stable secondary sort on caller index keeps tag-slice ordering
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// deterministic for tags with identical byte offsets.
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var sorted = group
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.OrderBy(t => t.StartByte)
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.ThenBy(t => t.CallerIndex)
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.ToList();
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var blockStart = sorted[0].StartByte;
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var blockEnd = sorted[0].StartByte + sorted[0].ByteCount;
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var blockSlices = new List<TagSlice>
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{
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new(sorted[0].CallerIndex, 0, sorted[0].ByteCount),
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};
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for (var i = 1; i < sorted.Count; i++)
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{
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var t = sorted[i];
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var gap = t.StartByte - blockEnd;
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// gap < 0 means the next tag overlaps with the current block — treat
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// as zero-gap merge (overlap is fine, the slice just reuses earlier
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// bytes). gap <= threshold = merge; otherwise close the current
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// block and start a new one.
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if (gap <= gapMergeBytes)
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{
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var newEnd = Math.Max(blockEnd, t.StartByte + t.ByteCount);
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blockSlices.Add(new TagSlice(t.CallerIndex, t.StartByte - blockStart, t.ByteCount));
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blockEnd = newEnd;
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}
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else
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{
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ranges.Add(new BlockReadRange(
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group.Key.Area, group.Key.DbNumber, blockStart, blockEnd - blockStart, blockSlices));
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blockStart = t.StartByte;
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blockEnd = t.StartByte + t.ByteCount;
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blockSlices = [new TagSlice(t.CallerIndex, 0, t.ByteCount)];
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}
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}
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ranges.Add(new BlockReadRange(
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group.Key.Area, group.Key.DbNumber, blockStart, blockEnd - blockStart, blockSlices));
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}
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return ranges;
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}
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/// <summary>
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/// True when <paramref name="tag"/>'s on-wire width is variable / header-prefixed.
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/// Such tags MUST NOT participate in block coalescing because the slice into a
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/// coalesced byte buffer would land at a wrong offset for any neighbour.
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/// </summary>
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internal static bool IsOpaqueSize(S7TagDefinition tag)
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{
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// Variable-width string types — STRING/WSTRING carry a 2-byte (or 4-byte)
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// header and the actual length depends on the runtime value, not the
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// declared StringLength. CHAR/WCHAR are fixed-width (1 / 2 bytes) but
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// routed via the per-tag string codec path, so coalescing them would
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// bypass the codec; treat them as opaque to keep the decode surface
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// unchanged.
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if (tag.DataType is S7DataType.String or S7DataType.WString
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or S7DataType.Char or S7DataType.WChar)
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return true;
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// Structured timestamps — DTL is 12 bytes, DT is 8 bytes BCD-encoded;
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// both decode through S7DateTimeCodec and would silently mis-decode if
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// the slice landed mid-block. S5TIME/TIME/TOD/DATE are fixed-width 2/4
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// bytes but currently flow through the per-tag codec path; treat them
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// all as opaque so the planner emits a single-tag range and the existing
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// codec dispatch stays the source of truth for date/time decode.
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if (tag.DataType is S7DataType.Dtl or S7DataType.DateAndTime
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or S7DataType.S5Time or S7DataType.Time or S7DataType.TimeOfDay or S7DataType.Date)
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return true;
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// Arrays opt out: per-tag width is N × elementBytes, the slice must be
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// exact. Routing them as opaque keeps the array-aware byte-range read
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// path in S7Driver.ReadOneAsync.
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if (tag.ElementCount is int n && n > 1)
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return true;
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return false;
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}
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/// <summary>
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/// Byte width of a packable scalar tag for byte-range coalescing. Mirrors the
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/// size suffix the address grammar carried (<see cref="S7Size.Bit"/>=1 byte
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/// because reading a single bit still requires reading the containing byte;
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/// bit-extraction happens in the slice step).
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/// </summary>
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internal static int ScalarByteCount(S7Size size) => size switch
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{
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S7Size.Bit => 1,
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S7Size.Byte => 1,
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S7Size.Word => 2,
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S7Size.DWord => 4,
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S7Size.LWord => 8,
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_ => throw new InvalidOperationException($"Unknown S7Size {size}"),
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};
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}
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Reference in New Issue
Block a user