Requirements Legend

M = must haves
S = should have
C = could have
W = won't have
dipsplay tip at the start of the game
- M: click on 'x' to close tip modal
- M: persist user state in localStorage to hide tip on refresh
user can select cell by clicking
- M: in UI: highlight all columns, rows and subgrids that contain user selected cell
  - M: 1) if cell is empty, only show highlights as listed above
  - C: 2) if cell contains a number highlight all cells that contain the same number
user enters a number by keyboard
- M: if number is valid - highlight as default
- S: if number is invalid - highlight all matching numbers in all columns, rows and subgrids that contain the user's number
  - display incorrect input in red font, red background for matching colors
S: user notes
- entering number adds a node to the notes subgrid
  - numbers always go in the same place, 1 --> coord (0,0)
  - entering an existing number removes it from UI
  - entering a guess where notes exists erases all notes
C: arrow keys navigation
M: on completion a message is displayed
M: button that displays a menu to select a new game or to restart
- S: spinner displays while game is being rendered
M: game header displays current difficulty
S: restart button
S: erase button

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