assign←{⎕ML←1                               ⍝ Hungarian method cost assignment.

    step0←{step1(⌽⌈\⌽⍴⍵)↑⍵}                 ⍝ 0: at least as many rows as cols.

    step1←{step2(↓⍵)-⌊/⍵}                  ⍝ 1: reduce rows by minimum value.

    step2←{                                 ⍝ 2: mark independent zeros.
        stars←{                             ⍝ independent zeros.
            ~1∊⍵:⍺                          ⍝ no more zeros: done.
            next←<\<⍀⍵                      ⍝ more independent zeros.
            mask(rows next)cols next      ⍝ mask of dependent rows and cols.
            (⍺∨next)∇ ⍵>mask                ⍝ ⍺-accumulated star matrix.
        }                                   ⍝
        zeros←{⍵+0 stars ⍵}⍵=0              ⍝ 1=>zero, 2=>independent zero.
        step3zeros                       ⍝ next step: 3.
    }

    step3←{costs zeros←⍵                    ⍝ 3: cover cols with starred zeros.
        starszeros=2                       ⍝ starred zeros.
        covers←2×cols stars                 ⍝ covered cols.
        ~0∊covers:stars                     ⍝ all cols covered: solution.
        step4 costs zeros covers            ⍝ next step: 4.
    }

    step4←{costs zeros covers←⍵             ⍝ 4: adjust covering lines.
        maskcovers=0                       ⍝ mask of uncovered elements.
        open←1=mask×zeros                   ⍝ uncovered zeros.
        ~1∊open:(⌊/(,mask)/,costs)step6 ⍵   ⍝ no uncovered zeros, next step :6.
        primefirst open                    ⍝ choose first uncovered zero.
        prowrows prime                     ⍝ row containing prime.
        star←2=zeros×prow                   ⍝ star in row containing prime.
        ~1∊star:prime step5{                ⍝ no star in row, next step :5,
            costsprime                   ⍝ adjusted zeros matrix,
        }zeros+2×prime                      ⍝ new primed zero (3).
        cnextcovers+prow-2×cols star       ⍝ adjusted covers.
        znextzeros⌈3×prime                 ⍝ primed zero.
        ∇ costs znext cnext                 ⍝ adjusted zeros and covers
    }

    step5←{costs zeros prime←⍵              ⍝ 5: exchange starred zeros.
        star(cols prime)zeros=2           ⍝ next star.
        ~1∊star:step3 ⍺{                    ⍝ no stars: next step :3.
            {costs ⍵}{⍵-2×⍵=3}⍵-⍺∧⍵>1       ⍝ unstarred stars; starred primes.
        }zeros                              ⍝ adjusted zero markers.
        pnext(rows star)zeros=3           ⍝ next prime.
        (⍺∨pnextstar)costs zeros pnext   ⍝ ⍺-accumulated prime-star-··· path.
    }

    step6←{costs zeros covers←⍵             ⍝ 6: adjust cost matrix.
        cnextcosts+⍺ׯ1 1+.×0 3∘.=covers   ⍝ add and subtract minimum value.
        znextzeros+(×costs)cnext         ⍝ amended zeros marker.
        step4 cnext znext covers            ⍝ next step: 4.
    }

    rows←{(⍴⍵)(⊃⌽⍴⍵)/∨/⍵}                  ⍝ row propagation.
    cols←{(⍴⍵)⍴∨⌿⍵}                         ⍝ column propagation.
    first←{(⍴⍵)⍴<\,⍵}                       ⍝ first 1 in bool matrix.

    (⍴⍵)step0 ⍵                            ⍝ start with step 0.
}
code_colours

test script

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