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/-
git log --format=%ct | lake exe commitgraph

Sample output:

2025-04-20 ▒   ▓
2025-04-27   ▒
2025-05-04   ▓  ▓▒
2025-05-11    ▒▓
2025-05-18  ▒   █▓
2025-05-25 █▒
2025-06-01
2025-06-08
2025-06-15
2025-06-22
2025-06-29    ▒
2025-07-06
2025-07-13 ▓
2025-07-20
2025-07-27      ▒
2025-08-03 ▒
2025-08-10    ▓▓ ▓
2025-08-17
2025-08-24 ▓▓    █
2025-08-31 ▒ ▒▓▒▒▒
2025-09-07 ▒▒███
2025-09-14  ▒▒▒
2025-09-21 ▒
2025-09-28 ▒█  ▒
2025-10-05    ▓ ▓▓
2025-10-12 █▓
-/

import Std.Time
open Std.Time

instance : LE PlainDate := leOfOrd

def main : IO Unit := do
  let stdin ← IO.getStdin
  let lines := (← stdin.readToEnd).splitOn "\n" |>.toArray |>.filterMap λ x ↦
    (Timestamp.toPlainDateAssumingUTC ∘ Timestamp.ofSecondsSinceUnixEpoch ∘ Second.Offset.ofNat) <$> x.toNat?
  let sorted_lines := lines.qsortOrd
  if h : 0 < sorted_lines.size then
    let mut daily := #[]
    let mut date := sorted_lines[0]
    let mut i := 0
    while date ≤ sorted_lines[sorted_lines.size - 1] do
      let mut cnt := 0
      while if h : i < sorted_lines.size then sorted_lines[i] = date else false do
        cnt := cnt + 1
        i := i + 1
      daily := daily.push cnt
      date := date.addDays 1
    let m := daily.toList.max?.getD 0
    let colors := [" ", "░", "▒", "▓", "█"]
    i := 0
    date := sorted_lines[0]
    for h : cnt in daily do
      if i % 7 = 0 then
        if i > 0 then IO.println ""
        IO.print s!"{date} "
        date := date.addDays 7
      IO.print <| colors[if cnt = 0 then 0 else if cnt < m / 8 then 1 else if cnt < m / 4 then 2 else if cnt < m / 2 then 3 else 4]'(by grind)
      i := i + 1
    IO.println ""