add another secondary constructor to FilePermissions to read permissions from bytes

1 files changed, 29 insertions, 2 deletions

diff --git a/src/main/kotlin/FilePermissions.kt b/src/main/kotlin/FilePermissions.kt
index dbb9dc4..ce25d23 100644
--- a/src/main/kotlin/FilePermissions.kt
+++ b/src/main/kotlin/FilePermissions.kt
@@ -23,6 +23,8 @@ class FilePermissions {
      */
     val isDirectory: Boolean
 
+    private val DIRECTORY_BIT: UInt = 0x80000000u
+
     enum class Permissions(val bits: UByte) {
         READ(0x4u),
         WRITE(0x2u),
@@ -30,7 +32,16 @@ class FilePermissions {
         READ_WRITE(READ.bits.or(WRITE.bits)),
         READ_EXECUTE(READ.bits.or(EXECUTE.bits)),
         WRITE_EXECUTE(WRITE.bits.or(EXECUTE.bits)),
-        READ_WRITE_EXECUTE(READ.bits.or(WRITE.bits.or(EXECUTE.bits)))
+        READ_WRITE_EXECUTE(READ.bits.or(WRITE.bits.or(EXECUTE.bits)));
+
+        companion object {
+            /**
+             * Obtain a [Permissions] instance by matching its value.
+             *
+             * @throws NoSuchElementException if no such element has the provided value.
+             */
+            fun fromByte(bits: UByte) = Permissions.entries.first { it.bits == bits }
+        }
     }
 
     /**
@@ -49,6 +60,22 @@ class FilePermissions {
     }
 
     /**
+     * Constructor for raw file permission data. Only the first 4 elements are read.
+     *
+     * @param raw The raw file permission data.
+     * @throws IllegalArgumentException if [raw] does not have at least 4 elements.
+     */
+    constructor(raw: List<UByte>) {
+        require(raw.size >= 4)
+        val raw = raw.slice(0..4)
+        val dirValue = raw[0].toUInt().xor(this.DIRECTORY_BIT)
+        this.isDirectory = dirValue > 0u
+        this.userPerms = Permissions.fromByte(raw[1])
+        this.groupPerms = Permissions.fromByte(raw[2])
+        this.elsePerms = Permissions.fromByte(raw[3])
+    }
+
+    /**
      * Turn the permissions described by the [FilePermissions] fields into a permission integer (4 bytes).
      */
     fun toPermissionInt(): UInt {
@@ -58,7 +85,7 @@ class FilePermissions {
             perms.or(permFileds[i].bits.toUInt().shl(8 * (permFileds.size - 1 - i)))
         }
         if (isDirectory) {
-            perms.or(0x80000000u)
+            perms.or(this.DIRECTORY_BIT)
         }
         return perms
     }