Update contains method to include start position (#34)

[tothambrus11]

Fixes #34

[mattmassicotte]

Ahh yes, this is a tricky one. Ranges are always subtle. I was planning on adding some testing around this to make sure I understood what "correct" was. What do you think about that?

[tothambrus11]

Ranges are half-open, as specified by the LSP. When implementing language servers to resolve a node under a cursor, we would typically not use this, but something that matches on both ends inclusively though. E.g. if we have an identifier x, the cursor should match on both sides of it.

import FrontEnd
import Logging

extension Program {

  /// - Requires: The source file of `position` is present in `self`.
  public func innermostTree(
    containing position: SourcePosition, reportingLogsTo logger: Logger, in f: SourceFile.ID
  ) -> AnySyntaxIdentity? {
    var v = NodeFinder(position)
    visit(topLevelDeclarations(in: f), calling: &v)
    return v.deepestMatch
  }

}

/// Requires that the visiting happens in a depth-first order.
private struct NodeFinder: SyntaxVisitor {

  // todo use binary search for efficiency if we can assume AST entries are sorted by position (probably they aren't though)

  let targetPosition: SourcePosition
  private(set) var deepestMatch: AnySyntaxIdentity?
  private var deepestMatchDepth: Int = -1
  private var currentDepth: Int = 0

  public init(_ targetPosition: SourcePosition) {
    self.targetPosition = targetPosition
  }

  mutating func willEnter(_ n: AnySyntaxIdentity, in program: Program) -> Bool {
    if program[n].site.region.containsInclusive(targetPosition.index) {
      if currentDepth > deepestMatchDepth {
        deepestMatchDepth = currentDepth
        deepestMatch = n
      }
    } else {
      if program.isScope(n) {
        // If it's a scope, we know its children's sites are strictly subsumed, so we can skip them.
        return false
      }
    }

    currentDepth += 1

    return true  // continue visiting children
  }

  public mutating func willExit(_ node: AnySyntaxIdentity, in program: Program) {
    currentDepth -= 1
  }

}

extension Range {

  func containsInclusive(_ i: Bound) -> Bool {
    return self.lowerBound <= i && i <= self.upperBound
  }

}