Add transpose command for NSV data

README.md

@@ -49,6 +49,20 @@ The `validate` command:
 - with `--table`, checks that all rows have equal length
 - Exits with 0 on success, 1 on warnings or errors
 
+### Transpose
+
+Transpose a table.  
+Both `t` and `transpose` are valid, prefer the latter for long-living scripts.
+
+```sh
+nsv t table.nsv
+cat table.nsv | nsv t
+```
+
+The `transpose` command:
+- Requires table input (all rows must have equal arity), errors on ragged data
+- Allocates full input in memory (once, but still)
+
 ### Stats
 
 Structural overview of an NSV file.

src/main.rs

@@ -41,6 +41,15 @@ enum Commands {
         #[arg(value_name = "FILE")]
         file: Option<String>,
     },
+
+    /// Transpose rows and columns of NSV data
+    #[command(alias = "t")]
+    Transpose {
+        /// Input file (reads from stdin if omitted or "-")
+        #[arg(value_name = "FILE")]
+        file: Option<String>,
+    },
+
 }
 
 fn main() {
@@ -63,6 +72,12 @@ fn main() {
         Commands::Stats { file } => {
             stats(file);
         }
+        Commands::Transpose { file } => {
+            if let Err(e) = transpose(file) {
+                eprintln!("error: {}", e);
+                std::process::exit(1);
+            }
+        }
     }
 }
 
@@ -295,3 +310,52 @@ fn validate(file: Option<String>, table: bool) -> i32 {
 
     exit_code
 }
+
+/// Transpose rows and columns of NSV data.
+///
+/// Requires table input (all rows must have equal arity).
+/// Errors on ragged data.
+fn transpose(file: Option<String>) -> Result<(), String> {
+    let raw = read_input(&file);
+
+    // Split into raw lines (already-escaped cell bytes).
+    // Group by blank lines into rows of raw cell slices.
+    let mut rows: Vec<Vec<&[u8]>> = Vec::new();
+    let mut current_row: Vec<&[u8]> = Vec::new();
+    for line in raw.split(|&b| b == b'\n') {
+        if line.is_empty() {
+            if !current_row.is_empty() {
+                rows.push(current_row);
+                current_row = Vec::new();
+            }
+        } else {
+            current_row.push(line);
+        }
+    }
+    if !current_row.is_empty() {
+        rows.push(current_row);
+    }
+
+    if rows.is_empty() {
+        return Ok(());
+    }
+
+    let arity = rows[0].len();
+    if rows.iter().any(|r| r.len() != arity) {
+        return Err("not a table — row arities differ".to_string());
+    }
+    if arity == 0 {
+        return Ok(());
+    }
+
+    let mut out = io::BufWriter::new(io::stdout().lock());
+    for col in 0..arity {
+        for row in &rows {
+            out.write_all(row[col]).map_err(|e| e.to_string())?;
+            out.write_all(b"\n").map_err(|e| e.to_string())?;
+        }
+        out.write_all(b"\n").map_err(|e| e.to_string())?;
+    }
+    Ok(())
+}
+

tests/test_transpose.sh

@@ -0,0 +1,149 @@
+#!/bin/bash
+
+cd "$(dirname "$0")/.."
+cargo build --quiet || exit 1
+
+TMPDIR=$(mktemp -d)
+trap 'rm -rf "$TMPDIR"' EXIT
+
+PASS=0
+FAIL=0
+
+# run_test NAME STDIN_OR_FILE EXPECTED_STDOUT [USE_STDIN]
+#   Runs transpose, checks exit 0, compares stdout exactly.
+run_test() {
+    local name="$1"
+    local input="$2"
+    local expected="$3"
+    local use_stdin="$4"
+
+    local stdout stderr exit_code
+
+    if [[ "$use_stdin" == "stdin" ]]; then
+        stdout=$(printf '%s' "$input" | cargo run --quiet -- transpose 2>$TMPDIR/stderr) && exit_code=0 || exit_code=$?
+    else
+        stdout=$(cargo run --quiet -- transpose "$input" 2>$TMPDIR/stderr) && exit_code=0 || exit_code=$?
+    fi
+    stderr=$(cat $TMPDIR/stderr)
+    rm -f $TMPDIR/stderr
+
+    local failed=0
+
+    if [[ "$exit_code" -ne 0 ]]; then
+        echo "FAIL: $name - expected exit 0, got $exit_code"
+        failed=1
+    fi
+
+    if [[ -n "$stderr" ]]; then
+        echo "FAIL: $name - unexpected stderr: $stderr"
+        failed=1
+    fi
+
+    if [[ "$stdout" != "$expected" ]]; then
+        echo "FAIL: $name - stdout mismatch"
+        diff <(echo "$stdout") <(echo "$expected")
+        failed=1
+    fi
+
+    if [[ "$failed" -eq 0 ]]; then
+        echo "PASS: $name"
+        PASS=$((PASS + 1))
+    else
+        FAIL=$((FAIL + 1))
+    fi
+}
+
+echo "Running transpose tests..."
+echo
+
+F=tests/fixtures
+
+# ── Empty input ──
+
+run_test "empty file" $F/empty.nsv ""
+
+# ── 2×2 table → 2×2 table ──
+
+run_test "2x2 table" $F/table.nsv "$(printf 'a\nc\n\nb\nd\n\n')"
+
+# ── 3×4 table → 4×3 table ──
+
+run_test "3x4 table" $F/table_3x4.nsv "$(printf 'a\ne\ni\n\nb\nf\nj\n\nc\ng\nk\n\nd\nh\nl\n\n')"
+
+# ── Ragged rows rejected ──
+
+cargo run --quiet -- transpose $F/ragged.nsv >/dev/null 2>$TMPDIR/stderr && exit_code=0 || exit_code=$?
+stderr=$(cat $TMPDIR/stderr)
+rm -f $TMPDIR/stderr
+if [[ "$exit_code" -eq 1 ]] && [[ "$stderr" == *"not a table"* ]]; then
+    echo "PASS: ragged rows rejected"
+    PASS=$((PASS + 1))
+else
+    echo "FAIL: ragged rows rejected - exit=$exit_code stderr=$stderr"
+    FAIL=$((FAIL + 1))
+fi
+
+# ── Single row → single column ──
+# Input: one row with 3 cells [x,y,z]
+
+run_test "single row to column" "$(printf 'x\ny\nz\n\n')" "$(printf 'x\n\ny\n\nz\n\n')" "stdin"
+
+# ── Single column → single row ──
+# Input: two rows each with one cell
+run_test "single column to row" "$(printf 'p\n\nq\n\n')" "$(printf 'p\nq\n\n')" "stdin"
+
+# ── stdin input ──
+
+run_test "stdin" "$(printf 'a\nb\n\nc\nd\n\n')" "$(printf 'a\nc\n\nb\nd\n\n')" "stdin"
+
+# ── Roundtrip: transpose(transpose(x)) == x ──
+
+# Use table.nsv: transpose twice should recover original
+first=$(cargo run --quiet -- transpose $F/table.nsv 2>/dev/null)
+second=$(printf '%s' "$first" | cargo run --quiet -- transpose 2>/dev/null)
+original=$(cat $F/table.nsv)
+if [[ "$second" == "$original" ]]; then
+    echo "PASS: roundtrip transpose(transpose(x)) == x"
+    PASS=$((PASS + 1))
+else
+    echo "FAIL: roundtrip transpose(transpose(x)) == x"
+    diff <(echo "$second") <(echo "$original")
+    FAIL=$((FAIL + 1))
+fi
+
+# ── Roundtrip with 3×4 table ──
+
+first=$(cargo run --quiet -- transpose $F/table_3x4.nsv 2>/dev/null)
+second=$(printf '%s' "$first" | cargo run --quiet -- transpose 2>/dev/null)
+original=$(cat $F/table_3x4.nsv)
+if [[ "$second" == "$original" ]]; then
+    echo "PASS: roundtrip 3x4"
+    PASS=$((PASS + 1))
+else
+    echo "FAIL: roundtrip 3x4"
+    diff <(echo "$second") <(echo "$original")
+    FAIL=$((FAIL + 1))
+fi
+
+# ── Multiline cell (escaped content preserved) ──
+
+stdout=$(cargo run --quiet -- transpose $F/multiline_cell.nsv 2>/dev/null)
+exit_code=$?
+if [[ "$exit_code" -eq 0 ]] && [[ -n "$stdout" ]]; then
+    # Validate the transposed output is valid NSV
+    echo "$stdout" | cargo run --quiet -- validate 2>/dev/null && validate_exit=0 || validate_exit=$?
+    if [[ "$validate_exit" -eq 0 ]]; then
+        echo "PASS: multiline cell transposed is valid NSV"
+        PASS=$((PASS + 1))
+    else
+        echo "FAIL: multiline cell transposed is not valid NSV"
+        FAIL=$((FAIL + 1))
+    fi
+else
+    echo "FAIL: multiline cell transpose failed"
+    FAIL=$((FAIL + 1))
+fi
+
+echo
+echo "Results: $PASS passed, $FAIL failed"
+[[ "$FAIL" -eq 0 ]]