feat: replace text-based DXF hash with geometric content hash
SolidWorks re-exports produce files with identical geometry but different entity ordering, handle assignments, style names, and floating-point epsilon values. This caused hash mismatches and unnecessary API updates. Uses ACadSharp to parse DXF entities and build canonical, sorted signatures (LINE, ARC, CIRCLE, MTEXT) with coordinates rounded to 4 decimal places. Falls back to raw file hash if parsing fails. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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@@ -1,26 +1,32 @@
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using System;
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using System.Collections.Generic;
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using System.Globalization;
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using System.IO;
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using System.Linq;
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using System.Security.Cryptography;
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using System.Text;
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using ACadSharp.Entities;
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using ACadSharp.IO;
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namespace ExportDXF.Utilities
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{
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public static class ContentHasher
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{
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/// <summary>
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/// Computes a SHA256 hash of DXF file content, skipping the HEADER section
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/// which contains timestamps that change on every save.
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/// Computes a SHA256 hash of DXF geometry, ignoring entity ordering,
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/// handle assignments, style names, and floating-point epsilon differences
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/// that SolidWorks changes between re-exports of identical geometry.
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/// Falls back to a raw file hash if ACadSharp parsing fails.
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/// </summary>
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public static string ComputeDxfContentHash(string filePath)
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{
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var text = File.ReadAllText(filePath);
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var contentStart = FindEndOfHeader(text);
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var content = contentStart >= 0 ? text.Substring(contentStart) : text;
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using (var sha = SHA256.Create())
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try
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{
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var bytes = sha.ComputeHash(Encoding.UTF8.GetBytes(content));
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return BitConverter.ToString(bytes).Replace("-", "").ToLowerInvariant();
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return ComputeGeometricHash(filePath);
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}
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catch
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{
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return ComputeFileHash(filePath);
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}
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}
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@@ -37,82 +43,95 @@ namespace ExportDXF.Utilities
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}
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}
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/// <summary>
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/// Finds the position immediately after the HEADER section's ENDSEC marker.
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/// DXF HEADER format:
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/// 0\nSECTION\n2\nHEADER\n...variables...\n0\nENDSEC\n
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/// Returns -1 if no HEADER section is found.
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/// </summary>
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private static int FindEndOfHeader(string text)
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private static string ComputeGeometricHash(string filePath)
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{
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// Find the HEADER section start
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var headerIndex = FindGroupCode(text, 0, "2", "HEADER");
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if (headerIndex < 0)
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return -1;
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// Advance past the HEADER value line so pair scanning stays aligned
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var headerLineEnd = text.IndexOf('\n', headerIndex);
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if (headerLineEnd < 0)
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return -1;
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// Find the ENDSEC that closes the HEADER section
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var pos = headerLineEnd + 1;
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while (pos < text.Length)
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using (var reader = new DxfReader(filePath))
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{
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var endsecIndex = FindGroupCode(text, pos, "0", "ENDSEC");
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if (endsecIndex < 0)
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return -1;
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var doc = reader.Read();
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var signatures = new List<string>();
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// Move past the ENDSEC line
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var lineEnd = text.IndexOf('\n', endsecIndex);
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return lineEnd >= 0 ? lineEnd + 1 : text.Length;
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foreach (var entity in doc.Entities)
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{
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signatures.Add(GetEntitySignature(entity));
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}
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signatures.Sort(StringComparer.Ordinal);
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var combined = string.Join("\n", signatures);
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using (var sha = SHA256.Create())
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{
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var bytes = sha.ComputeHash(Encoding.UTF8.GetBytes(combined));
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return BitConverter.ToString(bytes).Replace("-", "").ToLowerInvariant();
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}
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}
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return -1;
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}
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/// <summary>
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/// Finds a DXF group code pair (code line followed by value line) starting from the given position.
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/// Returns the position of the value line, or -1 if not found.
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/// </summary>
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private static int FindGroupCode(string text, int startIndex, string groupCode, string value)
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private static string GetEntitySignature(Entity entity)
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{
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var pos = startIndex;
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while (pos < text.Length)
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var layer = entity.Layer?.Name ?? "";
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switch (entity)
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{
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// Skip whitespace/newlines to find the group code
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while (pos < text.Length && (text[pos] == '\r' || text[pos] == '\n' || text[pos] == ' '))
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pos++;
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case Line line:
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return GetLineSignature(line, layer);
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case Arc arc:
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return GetArcSignature(arc, layer);
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case Circle circle:
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return GetCircleSignature(circle, layer);
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case MText mtext:
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return GetMTextSignature(mtext, layer);
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default:
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return $"{entity.GetType().Name}|{layer}";
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}
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}
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if (pos >= text.Length)
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break;
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private static string GetLineSignature(Line line, string layer)
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{
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var p1 = FormatPoint(line.StartPoint.X, line.StartPoint.Y);
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var p2 = FormatPoint(line.EndPoint.X, line.EndPoint.Y);
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// Read the group code line
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var codeLineEnd = text.IndexOf('\n', pos);
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if (codeLineEnd < 0)
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break;
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var codeLine = text.Substring(pos, codeLineEnd - pos).Trim();
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// Move to the value line
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var valueStart = codeLineEnd + 1;
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if (valueStart >= text.Length)
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break;
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var valueLineEnd = text.IndexOf('\n', valueStart);
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if (valueLineEnd < 0)
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valueLineEnd = text.Length;
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var valueLine = text.Substring(valueStart, valueLineEnd - valueStart).Trim();
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if (codeLine == groupCode && string.Equals(valueLine, value, StringComparison.OrdinalIgnoreCase))
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return valueStart;
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// Move to the next pair
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pos = valueLineEnd + 1;
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// Normalize endpoint order so direction doesn't affect the hash
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if (string.Compare(p1, p2, StringComparison.Ordinal) > 0)
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{
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var tmp = p1;
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p1 = p2;
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p2 = tmp;
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}
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return -1;
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return $"LINE|{layer}|{p1}|{p2}";
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}
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private static string GetArcSignature(Arc arc, string layer)
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{
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var center = FormatPoint(arc.Center.X, arc.Center.Y);
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var r = R(arc.Radius);
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var sa = R(arc.StartAngle);
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var ea = R(arc.EndAngle);
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return $"ARC|{layer}|{center}|{r}|{sa}|{ea}";
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}
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private static string GetCircleSignature(Circle circle, string layer)
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{
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var center = FormatPoint(circle.Center.X, circle.Center.Y);
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var r = R(circle.Radius);
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return $"CIRCLE|{layer}|{center}|{r}";
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}
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private static string GetMTextSignature(MText mtext, string layer)
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{
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var point = FormatPoint(mtext.InsertPoint.X, mtext.InsertPoint.Y);
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var text = mtext.Value ?? "";
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return $"MTEXT|{layer}|{point}|{text}";
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}
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private static string R(double value)
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{
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return Math.Round(value, 4).ToString(CultureInfo.InvariantCulture);
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}
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private static string FormatPoint(double x, double y)
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{
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return $"{R(x)},{R(y)}";
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}
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}
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}
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