feat(nbd): implement display layer (Phase 4)

Add output formatting for tickets via display.rs:
- print_ticket / format_ticket: key-value table view
- print_ticket_json / format_ticket_json: pretty JSON
- print_list / format_list: compact summary table with header
- print_list_json / format_list_json: JSON array output

format_* variants return String for testability. Unit tests added
covering field presence, dependency joining, header row, and JSON
validity. All 26 tests pass.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

nbd/PLAN.md

@@ -74,7 +74,7 @@ File I/O and directory traversal using `async-std`.
 
 Output formatting.
 
-- [ ] Implement `print_ticket(ticket: &Ticket)`: full tabular display
+- [x] Implement `print_ticket(ticket: &Ticket)`: full tabular display
   ```
   ID:           a3f9c2
   Title:        Fix login bug
@@ -84,15 +84,16 @@ Output formatting.
   Type:         bug
   Dependencies: b7d41e, c9e823
   ```
-- [ ] Implement `print_ticket_json(ticket: &Ticket)`: pretty-printed JSON to stdout
+- [x] Implement `print_ticket_json(ticket: &Ticket)`: pretty-printed JSON to stdout
-- [ ] Implement `print_list(tickets: &[Ticket])`: short table
+- [x] Implement `print_list(tickets: &[Ticket])`: short table
   ```
   ID       PRI  TYPE     STATUS       TITLE
   a3f9c2   8    bug      in_progress  Fix login bug
   b7d41e   5    task     todo         Add rate limiting
   ```
-- [ ] Implement `print_list_json(tickets: &[Ticket])`: JSON array to stdout
+- [x] Implement `print_list_json(tickets: &[Ticket])`: JSON array to stdout
-- [ ] Unit tests: table output contains expected field values, JSON output is valid
+- [x] Added `format_ticket`, `format_ticket_json`, `format_list`, `format_list_json` internal functions for testability
+- [x] Unit tests: table output contains expected field values, JSON output is valid
 
 ---
 

nbd/src/display.rs

@@ -2,3 +2,161 @@
 //!
 //! Provides functions for rendering tickets and ticket lists as human-readable
 //! tables or machine-readable JSON, depending on the caller's preference.
+//!
+//! Each `print_*` function writes directly to stdout and is intended for use
+//! in command handlers. The corresponding `format_*` functions return a
+//! `String` and are provided primarily for testing and composition.
+
+use crate::ticket::{Status, Ticket, TicketType};
+
+// ── Column widths for the summary list table ─────────────────────────────────
+
+/// Width of the ID column (6-char hex + padding).
+const COL_ID: usize = 9;
+/// Width of the priority column.
+const COL_PRI: usize = 5;
+/// Width of the ticket-type column ("project" = 7 chars, +2 padding).
+const COL_TYPE: usize = 9;
+/// Width of the status column ("in_progress" = 11 chars, +2 padding).
+const COL_STATUS: usize = 13;
+/// Width of each label in the full ticket view ("Dependencies:" = 13 + 1 space).
+const LABEL_WIDTH: usize = 14;
+
+// ── Internal helpers ──────────────────────────────────────────────────────────
+
+/// Return the canonical display string for a [`Status`] variant.
+///
+/// The strings match the serde serialisation: `"todo"`, `"in_progress"`,
+/// `"done"`.
+fn status_str(status: &Status) -> &'static str {
+    match status {
+        Status::Todo => "todo",
+        Status::InProgress => "in_progress",
+        Status::Done => "done",
+    }
+}
+
+/// Return the canonical display string for a [`TicketType`] variant.
+///
+/// The strings match the serde serialisation: `"project"`, `"feature"`,
+/// `"task"`, `"bug"`.
+fn ticket_type_str(ticket_type: &TicketType) -> &'static str {
+    match ticket_type {
+        TicketType::Project => "project",
+        TicketType::Feature => "feature",
+        TicketType::Task => "task",
+        TicketType::Bug => "bug",
+    }
+}
+
+// ── Public formatting functions ───────────────────────────────────────────────
+
+/// Format a single ticket as a human-readable key–value table.
+///
+/// Each field is displayed on its own line with a label padded to
+/// [`LABEL_WIDTH`] characters.  An empty `Dependencies` field is rendered as
+/// an empty string rather than being omitted.
+///
+/// ```text
+/// ID:           a3f9c2
+/// Title:        Fix login bug
+/// Body:         Users cannot log in with email addresses containing +
+/// Priority:     8
+/// Status:       in_progress
+/// Type:         bug
+/// Dependencies: b7d41e, c9e823
+/// ```
+pub fn format_ticket(ticket: &Ticket) -> String {
+    let deps = ticket.dependencies.join(", ");
+    [
+        format!("{:<LABEL_WIDTH$}{}", "ID:", ticket.id),
+        format!("{:<LABEL_WIDTH$}{}", "Title:", ticket.title),
+        format!("{:<LABEL_WIDTH$}{}", "Body:", ticket.body),
+        format!("{:<LABEL_WIDTH$}{}", "Priority:", ticket.priority),
+        format!("{:<LABEL_WIDTH$}{}", "Status:", status_str(&ticket.status)),
+        format!(
+            "{:<LABEL_WIDTH$}{}",
+            "Type:",
+            ticket_type_str(&ticket.ticket_type)
+        ),
+        format!("{:<LABEL_WIDTH$}{}", "Dependencies:", deps),
+    ]
+    .join("\n")
+}
+
+/// Print a full tabular representation of a single ticket to stdout.
+pub fn print_ticket(ticket: &Ticket) {
+    println!("{}", format_ticket(ticket));
+}
+
+/// Format a single ticket as a pretty-printed JSON object.
+///
+/// The output is suitable for piping or machine consumption.
+pub fn format_ticket_json(ticket: &Ticket) -> String {
+    serde_json::to_string_pretty(ticket).expect("ticket serialisation must not fail")
+}
+
+/// Print a single ticket as pretty-printed JSON to stdout.
+pub fn print_ticket_json(ticket: &Ticket) {
+    println!("{}", format_ticket_json(ticket));
+}
+
+/// Format a slice of tickets as a short summary table with a header row.
+///
+/// Columns are `ID`, `PRI`, `TYPE`, `STATUS`, and `TITLE`, each left-aligned
+/// to a fixed width.  Rows are presented in the order given — callers are
+/// responsible for sorting before passing the slice.  An empty slice still
+/// produces the header row.
+///
+/// ```text
+/// ID       PRI  TYPE     STATUS       TITLE
+/// a3f9c2   8    bug      in_progress  Fix login bug
+/// b7d41e   5    task     todo         Add rate limiting
+/// ```
+pub fn format_list(tickets: &[Ticket]) -> String {
+    let mut out = format!(
+        "{:<width_id$}{:<width_pri$}{:<width_type$}{:<width_status$}{}",
+        "ID",
+        "PRI",
+        "TYPE",
+        "STATUS",
+        "TITLE",
+        width_id = COL_ID,
+        width_pri = COL_PRI,
+        width_type = COL_TYPE,
+        width_status = COL_STATUS,
+    );
+
+    for ticket in tickets {
+        out.push('\n');
+        out.push_str(&format!(
+            "{:<width_id$}{:<width_pri$}{:<width_type$}{:<width_status$}{}",
+            ticket.id,
+            ticket.priority,
+            ticket_type_str(&ticket.ticket_type),
+            status_str(&ticket.status),
+            ticket.title,
+            width_id = COL_ID,
+            width_pri = COL_PRI,
+            width_type = COL_TYPE,
+            width_status = COL_STATUS,
+        ));
+    }
+
+    out
+}
+
+/// Print a short summary table of multiple tickets to stdout.
+pub fn print_list(tickets: &[Ticket]) {
+    println!("{}", format_list(tickets));
+}
+
+/// Format a slice of tickets as a pretty-printed JSON array.
+pub fn format_list_json(tickets: &[Ticket]) -> String {
+    serde_json::to_string_pretty(tickets).expect("ticket list serialisation must not fail")
+}
+
+/// Print a ticket list as a JSON array to stdout.
+pub fn print_list_json(tickets: &[Ticket]) {
+    println!("{}", format_list_json(tickets));
+}

nbd/src/tests.rs

@@ -288,3 +288,152 @@ mod store {
         assert_eq!(dir, Path::new("/tmp/project/.nbd/tickets"));
     }
 }
+
+// ── display module ────────────────────────────────────────────────────────────
+
+/// Tests for [`crate::display`].
+mod display {
+    use crate::display::{format_list, format_list_json, format_ticket, format_ticket_json};
+    use crate::ticket::{Status, Ticket, TicketType};
+
+    /// Build a fully-populated ticket for use in display tests.
+    fn sample_ticket() -> Ticket {
+        Ticket {
+            id: "a3f9c2".to_string(),
+            title: "Fix login bug".to_string(),
+            body: "Users cannot log in with email addresses containing +".to_string(),
+            priority: 8,
+            status: Status::InProgress,
+            dependencies: vec!["b7d41e".to_string(), "c9e823".to_string()],
+            ticket_type: TicketType::Bug,
+        }
+    }
+
+    /// `format_ticket` includes all field values in its output.
+    #[test]
+    fn format_ticket_contains_all_fields() {
+        let t = sample_ticket();
+        let output = format_ticket(&t);
+        assert!(output.contains("a3f9c2"), "should contain ID");
+        assert!(output.contains("Fix login bug"), "should contain title");
+        assert!(
+            output.contains("Users cannot log in"),
+            "should contain body"
+        );
+        assert!(output.contains('8'), "should contain priority");
+        assert!(output.contains("in_progress"), "should contain status");
+        assert!(output.contains("bug"), "should contain ticket type");
+        assert!(output.contains("b7d41e"), "should contain first dependency");
+        assert!(
+            output.contains("c9e823"),
+            "should contain second dependency"
+        );
+    }
+
+    /// `format_ticket` renders dependency IDs as a comma-separated list.
+    #[test]
+    fn format_ticket_joins_dependencies() {
+        let t = sample_ticket();
+        let output = format_ticket(&t);
+        assert!(
+            output.contains("b7d41e, c9e823"),
+            "dependencies should be comma-separated: {output}"
+        );
+    }
+
+    /// `format_ticket` renders an empty `Dependencies:` line when there are none.
+    #[test]
+    fn format_ticket_empty_dependencies() {
+        let t = Ticket::new("aaaaaa".to_string(), "No deps".to_string());
+        let output = format_ticket(&t);
+        assert!(
+            output.contains("Dependencies:"),
+            "Dependencies label should always appear"
+        );
+    }
+
+    /// `format_ticket_json` produces valid, parseable JSON containing key fields.
+    #[test]
+    fn format_ticket_json_is_valid_json() {
+        let t = sample_ticket();
+        let output = format_ticket_json(&t);
+        let parsed: serde_json::Value =
+            serde_json::from_str(&output).expect("output should be valid JSON");
+        assert_eq!(parsed["id"], "a3f9c2");
+        assert_eq!(parsed["title"], "Fix login bug");
+        assert_eq!(parsed["priority"], 8);
+        assert_eq!(parsed["status"], "in_progress");
+        assert_eq!(parsed["ticket_type"], "bug");
+    }
+
+    /// `format_list` includes a header row with all column names.
+    #[test]
+    fn format_list_shows_header() {
+        let output = format_list(&[]);
+        assert!(output.contains("ID"), "header should contain ID");
+        assert!(output.contains("PRI"), "header should contain PRI");
+        assert!(output.contains("TYPE"), "header should contain TYPE");
+        assert!(output.contains("STATUS"), "header should contain STATUS");
+        assert!(output.contains("TITLE"), "header should contain TITLE");
+    }
+
+    /// `format_list` includes every ticket's key fields.
+    #[test]
+    fn format_list_contains_all_tickets() {
+        let mut t1 = Ticket::new("id0001".to_string(), "First ticket".to_string());
+        t1.priority = 9;
+        let mut t2 = Ticket::new("id0002".to_string(), "Second ticket".to_string());
+        t2.priority = 3;
+        let tickets = vec![t1, t2];
+
+        let output = format_list(&tickets);
+        assert!(output.contains("id0001"), "should contain first ID");
+        assert!(
+            output.contains("First ticket"),
+            "should contain first title"
+        );
+        assert!(output.contains("id0002"), "should contain second ID");
+        assert!(
+            output.contains("Second ticket"),
+            "should contain second title"
+        );
+    }
+
+    /// `format_list` renders the correct status and type strings.
+    #[test]
+    fn format_list_renders_status_and_type_strings() {
+        let t = sample_ticket(); // status: in_progress, type: bug
+        let output = format_list(&[t]);
+        assert!(
+            output.contains("in_progress"),
+            "should render status string"
+        );
+        assert!(output.contains("bug"), "should render type string");
+    }
+
+    /// `format_list_json` produces a valid JSON array with one object per ticket.
+    #[test]
+    fn format_list_json_is_valid_json_array() {
+        let tickets = vec![
+            Ticket::new("id0001".to_string(), "First".to_string()),
+            Ticket::new("id0002".to_string(), "Second".to_string()),
+        ];
+        let output = format_list_json(&tickets);
+        let parsed: serde_json::Value =
+            serde_json::from_str(&output).expect("output should be valid JSON");
+        assert!(parsed.is_array(), "output should be a JSON array");
+        assert_eq!(parsed.as_array().unwrap().len(), 2);
+        assert_eq!(parsed[0]["id"], "id0001");
+        assert_eq!(parsed[1]["id"], "id0002");
+    }
+
+    /// `format_list_json` returns an empty JSON array for an empty slice.
+    #[test]
+    fn format_list_json_empty_slice() {
+        let output = format_list_json(&[]);
+        let parsed: serde_json::Value =
+            serde_json::from_str(&output).expect("output should be valid JSON");
+        assert!(parsed.is_array());
+        assert!(parsed.as_array().unwrap().is_empty());
+    }
+}