Pearl
Draw one loop that turns on every black pearl and runs straight through every white one
Arrow keys move, Enter starts and finishes a line, Shift with an arrow crosses an edge, Ctrl or ⌘ with an arrow lays a line and moves.
Click or tap near an edge between two squares to lay a line segment there, or to remove it. Drag through several squares to lay more of the loop at once, or drag over a laid part to remove it. Right-click, middle-click, Shift-click, Ctrl-click or long-press near an edge to mark it with a cross: you are sure the loop does not go through it. Arrow keys move; Enter starts a line and Enter again finishes it; Space, Escape or Backspace cancel it. Ctrl or ⌘ with an arrow lays or removes a line on that edge and moves. Shift with an arrow adds or removes a cross on that edge. Ctrl or ⌘ with Shift and an arrow lays or removes a line on that edge without moving. Ctrl or ⌘ Z undoes; Ctrl or ⌘ Shift Z or Ctrl or ⌘ Y redoes.
Progress saves on this device.
How to play
Draw lines between the centres of neighbouring squares so that they form a single closed loop. The loop does not have to pass through every square, and some pearls are the clues it must satisfy.
A black pearl is a corner, and neither of the squares next to it along the loop is also a corner. A white pearl is a straight piece, and at least one of the squares next to it along the loop is a corner. Only the squares the loop goes into from the pearl count; squares that merely touch it on the grid do not.
Mistakes show in red, never by colour alone: red segments are striped and a red-ringed pearl has a dashed edge. A square with more than two segments shows all of them in red. When there is a closed loop and other lines as well, the lines that are not part of the largest piece show in red. A pearl is ringed when the lines contradict it: a black pearl that the line crosses straight or that touches a corner beside it, a white pearl that the line turns on or that has straight lines on both sides, or a pearl whose line leads off the grid. When the lines form one closed loop, every pearl the loop misses is ringed as well. “Loop complete” stays once it has been reached along a line of play.
Undo and Redo follow your history; a new move after Undo replaces the undone moves. Restart clears the board and can be undone. Solve reveals the answer; a puzzle finished with Solve does not count as solved.
Engine: Simon Tatham’s Pearl, from the Portable Puzzle Collection. MIT license and credits.