Add password fingerprint identicon next to master password field

app/src/main/java/cz/bugsy/passwordzebra/ui/components/PasswordFingerprint.kt

@@ -0,0 +1,93 @@
+package cz.bugsy.passwordzebra.ui.components
+
+import androidx.compose.foundation.Canvas
+import androidx.compose.foundation.background
+import androidx.compose.foundation.border
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.size
+import androidx.compose.foundation.shape.RoundedCornerShape
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.remember
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.draw.clip
+import androidx.compose.ui.geometry.Offset
+import androidx.compose.ui.geometry.Size
+import androidx.compose.ui.graphics.Color
+import androidx.compose.ui.unit.Dp
+import androidx.compose.ui.unit.dp
+import java.security.MessageDigest
+
+/**
+ * Renders a small symmetric 5×5 identicon derived from the SHA-256 hash of [password].
+ * Color hue, saturation, and lightness are taken from hash bytes so that every unique
+ * password produces a visually distinct pattern that is easy to memorise.
+ *
+ * Shown as a plain surface-variant box when [password] is empty.
+ */
+@Composable
+fun PasswordFingerprint(
+    password: CharArray,
+    modifier: Modifier = Modifier,
+    size: Dp = 52.dp,
+) {
+    val surfaceVariant = MaterialTheme.colorScheme.surfaceVariant
+    val outline = MaterialTheme.colorScheme.outline
+
+    // Recompute hash only when the actual password content changes.
+    val hash: ByteArray? = if (password.isEmpty()) null else remember(String(password)) {
+        MessageDigest.getInstance("SHA-256")
+            .digest(String(password).toByteArray(Charsets.UTF_8))
+    }
+
+    val fillColor: Color? = hash?.let {
+        val hue = (it[0].toInt() and 0xFF) * 360f / 255f
+        val sat = 0.50f + (it[1].toInt() and 0xFF) / 255f * 0.35f
+        val lit = 0.38f + (it[2].toInt() and 0xFF) / 255f * 0.18f
+        Color.hsl(hue, sat, lit)
+    }
+
+    Box(
+        modifier = modifier
+            .size(size)
+            .clip(RoundedCornerShape(8.dp))
+            .background(surfaceVariant)
+            .border(1.dp, outline.copy(alpha = 0.35f), RoundedCornerShape(8.dp)),
+        contentAlignment = Alignment.Center,
+    ) {
+        if (hash != null && fillColor != null) {
+            val canvasSize = size * 0.80f   // leave a small margin inside the box
+            Canvas(modifier = Modifier.size(canvasSize)) {
+                val gridSize = 5
+                val cellW = this.size.width / gridSize
+                val cellH = this.size.height / gridSize
+                val gap = 1.5f
+
+                // Only decide cols 0..2; mirror cols 0..1 to cols 4..3.
+                // Col 2 is the centre column (not mirrored).
+                for (row in 0 until gridSize) {
+                    for (col in 0..2) {
+                        val bitIndex = row * 3 + col
+                        val filled = (hash[bitIndex / 8].toInt() ushr (bitIndex % 8)) and 1 == 1
+
+                        if (filled) {
+                            drawRect(
+                                color = fillColor,
+                                topLeft = Offset(col * cellW + gap, row * cellH + gap),
+                                size = Size(cellW - gap * 2, cellH - gap * 2),
+                            )
+                            if (col < 2) {
+                                drawRect(
+                                    color = fillColor,
+                                    topLeft = Offset((4 - col) * cellW + gap, row * cellH + gap),
+                                    size = Size(cellW - gap * 2, cellH - gap * 2),
+                                )
+                            }
+                        }
+                    }
+                }
+            }
+        }
+    }
+}

app/src/main/java/cz/bugsy/passwordzebra/ui/screens/DeterministicScreen.kt

@@ -66,6 +66,7 @@ import androidx.compose.ui.unit.dp
 import androidx.compose.ui.unit.sp
 import cz.bugsy.passwordzebra.R
 import cz.bugsy.passwordzebra.deterministic.DeterministicViewModel
+import cz.bugsy.passwordzebra.ui.components.PasswordFingerprint
 
 @OptIn(ExperimentalMaterial3Api::class)
 @Composable
@@ -278,13 +279,17 @@ fun DeterministicScreen(
 
             Spacer(Modifier.height(8.dp))
 
-            // Master password
+            // Master password + visual fingerprint
+            Row(
+                verticalAlignment = Alignment.CenterVertically,
+                modifier = Modifier.fillMaxWidth(),
+            ) {
                 OutlinedTextField(
                     value = String(state.masterPassword),
                     onValueChange = { viewModel.updateMasterPassword(it.toCharArray()) },
                     label = { Text(stringResource(R.string.det_master_password)) },
                     modifier = Modifier
-                    .fillMaxWidth()
+                        .weight(1f)
                         .focusRequester(masterPasswordFocus),
                     singleLine = true,
                     visualTransformation = if (state.masterPasswordVisible) VisualTransformation.None
@@ -309,6 +314,12 @@ fun DeterministicScreen(
                         }
                     },
                 )
+                Spacer(Modifier.width(8.dp))
+                PasswordFingerprint(
+                    password = state.masterPassword,
+                    size = 56.dp,
+                )
+            }
 
             Spacer(modifier = Modifier.height(4.dp))
         } // end Column