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package dotty.tools.pc

import java.net.URI
import java.util as ju

import dotty.tools.dotc.interactive.InteractiveDriver
import dotty.tools.dotc.reporting.Diagnostic
import dotty.tools.dotc.util.SourceFile

import scala.compiletime.uninitialized

/**
 * CachingDriver is a wrapper class that provides a compilation cache for InteractiveDriver.
 * CachingDriver skips running compilation if
 * - the target URI of `run` is the same as the previous target URI
 * - the content didn't change since the last compilation.
 *
 * This compilation cache enables Metals to skip compilation and re-use
 * the typed tree under the situation like developers
 * sequentially hover on the symbols in the same file without any changes.
 *
 * Note: we decided to cache only if the target URI is the same as in the previous run
 * because of `InteractiveDriver.currentCtx` that should return the context that
 * refers to the last compiled source file.
 * It would be ideal if we could update currentCtx even when we skip the compilation,
 * but we struggled to do that. See the discussion https://github.com/scalameta/metals/pull/4225#discussion_r941138403
 * To avoid the complexity related to currentCtx,
 * we decided to cache only when the target URI only if the same as the previous run.
 */
class CachingDriver(override val settings: List[String]) extends InteractiveDriver(settings):

  @volatile private var lastCompiledURI: URI = uninitialized

  private def alreadyCompiled(uri: URI, content: Array[Char]): Boolean =
    compilationUnits.get(uri) match
      case Some(unit)
          if lastCompiledURI == uri &&
            ju.Arrays.equals(unit.source.content(), content) =>
        true
      case _ => false

  override def run(uri: URI, source: SourceFile): List[Diagnostic] =
    val diags =
      if alreadyCompiled(uri, source.content) then Nil
      else super.run(uri, source)
    lastCompiledURI = uri
    diags

  override def run(uri: URI, sourceCode: String): List[Diagnostic] =
    val diags =
      if alreadyCompiled(uri, sourceCode.toCharArray().nn) then Nil
      else super.run(uri, sourceCode)
    lastCompiledURI = uri
    diags

end CachingDriver