1 files changed, 30 insertions, 0 deletions
diff --git a/Sources/NorgKit/Helpers/ASCIIByteSet.swift b/Sources/NorgKit/Helpers/ASCIIByteSet.swift
new file mode 100644
index 0000000..59efbe7
--- /dev/null
+++ b/Sources/NorgKit/Helpers/ASCIIByteSet.swift
@@ -0,0 +1,30 @@
+/// A membership test over the ASCII range (bytes `0`–`127`)
+/// We use this for performance, since markers are always ASCII, and this this
+/// faster than using Character.
+struct ASCIIByteSet {
+  private let low: UInt64
+  private let high: UInt64
+
+  /// Builds a set from the ASCII values.
+  init(_ characters: String) {
+    var lo: UInt64 = 0
+    var hi: UInt64 = 0
+    for byte in characters.utf8 {
+      if byte < 64 {
+        lo |= 1 << UInt64(byte)
+      } else if byte < 128 {
+        hi |= 1 << UInt64(byte - 64)
+      }
+    }
+    low = lo
+    high = hi
+  }
+
+  /// Whether `byte` is in the set.
+  @inline(__always)
+  func contains(_ byte: UInt8) -> Bool {
+    if byte < 64 { return low & (1 << UInt64(byte)) != 0 }
+    if byte < 128 { return high & (1 << UInt64(byte &- 64)) != 0 }
+    return false
+  }
+}