# fix: pad short rows in SetRows to the number of columns

## Problem

`SetRows` does not pad rows that have fewer cells than the table has columns.
As a result, short rows render fmt's missing-argument verb, and an empty row
combined with a first-column formatter panics:

```go
tbl := table.New("A", "B", "C").WithPadding(1)
tbl.SetRows([][]string{{"x", "y"}})
tbl.Print()
// A B C
// x y %!s(MISSING)
```

```go
tbl := table.New("A", "B").
	WithFirstColumnFormatter(func(f string, v ...interface{}) string { return fmt.Sprintf(f, v...) })
tbl.SetRows([][]string{{}})
tbl.Print() // panic: runtime error: index out of range [0] with length 0
```

`AddRow` already handles this: it allocates `make([]string, len(t.header))` per
row, so short input renders the remaining columns empty. The documented
behaviour ("Rows can have less cells than the total number of columns;
subsequent cells will be rendered empty.") is not honored by `SetRows`.

## Root cause

`table.go`, `SetRows`:

```go
for _, row := range rows {
	if len(row) > headerLength {
		t.rows = append(t.rows, row[:headerLength])
	} else {
		t.rows = append(t.rows, row) // short rows stored as-is
	}
}
```

Short rows are stored unchanged. Later, `printRow` builds one value per cell of
the row and passes them to `fmt.Fprintf` with a format string containing
`len(t.header)` verbs, so missing cells become `%!s(MISSING)`. With a
first-column formatter, `printRow` also indexes `vals[0]`, which panics for an
empty row.

## Fix

Pad short rows to the number of columns, mirroring `AddRow`:

```go
switch {
case len(row) > headerLength:
	t.rows = append(t.rows, row[:headerLength])
case len(row) < headerLength:
	// pad short rows so every column is rendered, matching AddRow
	padded := make([]string, headerLength)
	copy(padded, row)
	t.rows = append(t.rows, padded)
default:
	t.rows = append(t.rows, row)
}
```

## Test

Added `TestSetRowsShortRowsPadded` (asserts no `MISSING` verb and the exact
padded output) and `TestSetRowsEmptyRowWithFirstColumnFormatter` (asserts no
panic).

- Gate (`GOTOOLCHAIN=go1.25.10`): `go test ./...` -> ok (PASS, exit 0)
- `go vet ./...` -> exit 0
- `go build ./...` -> exit 0

Counterfactual: reverting `SetRows` makes `TestSetRowsShortRowsPadded` fail with
`output contains missing-arg verb: "A B C \nx y %!s(MISSING)\n"`.

Note: `gofmt -l table.go` reports the file, but this is pre-existing (the
`Print()` block near line 258 is space-indented on the base commit) and
unrelated to this change; the modified region is gofmt-clean, so unrelated lines
were intentionally left untouched.