<h1 align="center">
<img src="https://raw.githubusercontent.com/kantord/headson/main/docs/assets/logo.svg" alt="headson" width="221" />
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<p align="center">
<img src="https://raw.githubusercontent.com/kantord/headson/main/docs/assets/tapes/demo.gif" alt="Terminal demo" width="1560" height="900" />
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</p>
`head`/`tail` for JSON, YAML — but structure‑aware. Get a compact preview that shows both the shape and representative values of your data, all within a strict byte budget. (Just like `head`/`tail`, `hson` can also work with unstructured text files.)
Available as:
- CLI (see [Usage](#usage))
- Python library (see [Python Bindings](#python-bindings))
  
## Install
Using Cargo:
cargo install headson
> Note: the CLI installs as `hson`. All examples below use `hson ...`.
From source:
cargo build --release
target/release/hson --help
## Features
- Budgeted output: specify exactly how much you want to see
- Output formats: `auto | json | yaml | text`
- Styles: `strict | default | detailed`
- JSON family: `strict` → strict JSON, `default` → human‑friendly Pseudo, `detailed` → JS with inline comments
- YAML: always YAML; `strict` has no comments, `default` uses “# …”, `detailed` uses “# N more …”
- Text: prints raw lines. In `default` style, omissions are shown as a single line `…`; in `detailed`, as `… N more lines …`. `strict` omits array‑level summaries.
- Multiple inputs: preview many files at once with a shared or per‑file budget
- Fast: processes gigabyte‑scale files in seconds (mostly disk‑bound)
- Available as a CLI app and as a Python library
## Fits into command line workflows
If you’re comfortable with tools like `head` and `tail`, use `hson` when you want a quick, structured peek into a JSON file without dumping the entire thing.
- `head`/`tail` operate on bytes/lines - their output is not optimized for tree structures
- `jq`: you need to craft filters to preview large JSON files
- `hson`: head/tail for trees—zero‑config by default; force text with `-i text` when you want raw lines
## Usage
hson [FLAGS] [INPUT...]
- INPUT (optional, repeatable): file path(s). If omitted, reads from stdin. Multiple input files are supported.
- Prints the preview to stdout. On parse errors, exits non‑zero and prints an error to stderr.
Common flags:
- `-c, --bytes <BYTES>`: per‑file output budget (bytes). For multiple inputs, default total budget is `<BYTES> * number_of_inputs`.
- `-u, --chars <CHARS>`: per‑file output budget (Unicode code points). Behaves like `--bytes` but counts characters instead of bytes.
- `-C, --global-bytes <BYTES>`: total output budget across all inputs. With `--bytes`, the effective total is the smaller of the two.
- `-f, --format <auto|json|yaml|text>`: output format (default: `auto`).
- Auto: stdin → JSON family; filesets → per‑file based on extension (`.json` → JSON family, `.yaml`/`.yml` → YAML, unknown → Text).
- `-t, --template <strict|default|detailed>`: output style (default: `default`).
- JSON family: `strict` → strict JSON; `default` → Pseudo; `detailed` → JS with inline comments.
- YAML: always YAML; style only affects comments (`strict` none, `default` “# …”, `detailed` “# N more …”).
- `-i, --input-format <json|yaml|text>`: ingestion format (default: `json`). For filesets in `auto` format, ingestion is chosen by extensions.
- `-m, --compact`: no indentation, no spaces, no newlines
- `--no-newline`: single line output
- `--no-header`: suppress fileset section headers (useful when embedding output in scripts)
- `--no-space`: no space after `:` in objects
- `--indent <STR>`: indentation unit (default: two spaces)
- `--string-cap <N>`: max graphemes to consider per string (default: 500)
- `--head`: prefer the beginning of arrays when truncating (keep first N). Strings are unaffected. Display styles place omission markers accordingly; strict JSON remains unannotated. Mutually exclusive with `--tail`.
- `--tail`: prefer the end of arrays when truncating (keep last N). Strings are unaffected. Display styles place omission markers accordingly; strict JSON remains unannotated. Mutually exclusive with `--head`.
Notes:
- Multiple inputs:
- With newlines enabled, file sections are rendered with human‑readable headers (pass `--no-header` to suppress them). In compact/single‑line modes, headers are omitted.
- Order: inputs are sorted by git frecency (via frecenfile) when available, then by mtime; pass `--no-sort` to keep the original input order without repo scanning.
- In `--format auto`, each file uses its own best format: JSON family for `.json`, YAML for `.yaml`/`.yml`.
- Unknown extensions are treated as Text (raw lines) — safe for logs and `.txt` files.
- `--global-bytes` may truncate or omit entire files to respect the total budget.
- The tool finds the largest preview that fits the budget; even if extremely tight, you still get a minimal, valid preview.
- Directories and binary files are ignored; a notice is printed to stderr for each. Stdin reads the stream as‑is.
- Head vs Tail sampling: these options bias which part of arrays are kept before rendering. Display styles may still insert internal gap markers to honor very small budgets; strict JSON stays unannotated.
### Working with multiple files
- Budgets: per-file caps (`--bytes`/`--chars`/`--lines`) apply to each input; global caps (`--global-*`) constrain the combined output. Default byte budget scales by input count when no globals are set.
- Sorting: inputs are pre-sorted by git frecency (frecenfile) with last-modified-time fallback so recently touched files appear first. Pass `--no-sort` to preserve the order you provided and skip repo scanning.
- Headers: fileset sections get `==>` headers when newlines are enabled; hide them with `--no-header`. Compact and single-line modes omit headers automatically.
- Formats: in `--format auto`, each file picks JSON/YAML/Text based on extension; unknowns fall back to Text so mixed filesets “just work.”
## Budget Modes
- Bytes (`-c/--bytes`, `-C/--global-bytes`)
- Measures UTF‑8 bytes in the output.
- Default per‑file budget is 500 bytes when neither `--lines` nor `--chars` is provided.
- Multiple inputs: total default budget is `<BYTES> * number_of_inputs`; `--global-bytes` caps the total.
- Characters (`-u/--chars`)
- Measures Unicode code points (not grapheme clusters).
- Lines (`-n/--lines`, `-N/--global-lines`)
- Caps the number of lines in the output.
- Incompatible with `--no-newline`.
- Multiple inputs: defaults to `<LINES> * number_of_inputs`; `--global-lines` caps the total.
- Fileset headers, blank separators, and summary lines do not count toward the line cap; only actual content lines are considered.
- Interactions and precedence
- All active budgets are enforced simultaneously. The render must satisfy all of: bytes (if set), chars (if set), and lines (if set). The strictest cap wins.
- When only lines are specified, no implicit byte cap applies. When neither lines nor chars are specified, a 500‑byte default applies.
Quick one‑liners:
- Peek a big JSON stream (keeps structure):
zstdcat huge.json.zst | hson -c 800 -f json -t default
- Many files with a fixed overall size:
hson -C 1200 -f json -t strict logs/*.json
- Glance at a file, JavaScript‑style comments for omissions:
hson -c 400 -f json -t detailed data.json
- YAML with detailed comments:
hson -c 400 -f yaml -t detailed config.yaml
### Text mode
- Single file (auto):
hson -c 200 notes.txt
- Force Text ingest/output (useful when mixing with other extensions, or when the extension suggests JSON/YAML):
hson -c 200 -i text -f text notes.txt
# Force text ingest even if the file looks like JSON
hson -i text notes.json
- Styles on Text:
- default: omission as a standalone `…` line.
- detailed: omission as `… N more lines …`.
- strict: no array‑level omission line (individual long lines may still truncate with `…`).
> **Note:** Filesets always render with per-file auto templates. When you need to preview a directory of mixed formats, skip `-f text` and let `-f auto` pick the right renderer for each entry.
Show help:
hson --help
Note: flags align with head/tail conventions (`-c/--bytes`, `-C/--global-bytes`).
## Examples: head vs hson
Input:
```json
{"users":[{"id":1,"name":"Ana","roles":["admin","dev"]},{"id":2,"name":"Bo"}],"meta":{"count":2,"source":"db"}}
```
Naive cut (can break mid‑token):
```bash
jq -c . users.json | head -c 80
# {"users":[{"id":1,"name":"Ana","roles":["admin","dev"]},{"id":2,"name":"Bo"}],"me
```
Structured preview with hson (JSON family, default style → Pseudo):
```bash
hson -c 120 -f json -t default users.json
# {
# users: [
# { id: 1, name: "Ana", roles: [ "admin", … ] },
# …
# ]
# meta: { count: 2, … }
# }
```
Machine‑readable preview (JSON family, strict style → strict JSON):
```bash
hson -c 120 -f json -t strict users.json
# {"users":[{"id":1,"name":"Ana","roles":["admin"]}],"meta":{"count":2}}
```
## Terminal Demos
Regenerate locally:
- Place tapes under docs/tapes (e.g., docs/tapes/demo.tape)
- Run: cargo make tapes
- Outputs are written to docs/assets/tapes
## Python Bindings
A thin Python extension module is available on PyPI as `headson`.
- Install: `pip install headson` (ABI3 wheels for Python 3.10+ on Linux/macOS/Windows).
- API:
- `headson.summarize(text: str, *, format: str = "auto", style: str = "default", input_format: str = "json", byte_budget: int | None = None, skew: str = "balanced") -> str`
- `format`: `"auto" | "json" | "yaml"` (auto maps to JSON family for single inputs)
- `style`: `"strict" | "default" | "detailed"`
- `input_format`: `"json" | "yaml"` (ingestion)
- `byte_budget`: maximum output size in bytes (default: 500)
- `skew`: `"balanced" | "head" | "tail"` (affects display styles; strict JSON remains unannotated)
Examples:
```python
import json
import headson
data = {"foo": [1, 2, 3], "bar": {"x": "y"}}
preview = headson.summarize(json.dumps(data), format="json", style="strict", byte_budget=200)
print(preview)
# Prefer the tail of arrays (annotations show with style="default"/"detailed")
print(
headson.summarize(
json.dumps(list(range(100))),
format="json",
style="detailed",
byte_budget=80,
skew="tail",
)
)
# YAML support
doc = "root:\n items: [1,2,3,4,5,6,7,8,9,10]\n"
print(headson.summarize(doc, format="yaml", style="default", input_format="yaml", byte_budget=60))
```
## Source Code Support
Source code support is a challenging area. While `headson`'s algorithm and code structure would allow for the use of
completely accurate parsing using language-specific parsers using `tree-sitter`, this would increase the complexity
of the application and its number of dependencies.
Instead of attempting a deep parse of source code files, we convert them into nested arrays based on a heuristic that
understands indentation patterns in the file.
When `headson` detects a code-like file, it uses a set of additional heuristics:
- **Atomic line ingest**: each line is treated as an atomic string so omission markers never split a code line.
- **Depth-aware sampling**:
- We attempt to include more of the top level of the source code in order to give a good overview of classes, function and constants at the top level.
- Nested blocks (function bodies, loops) prefer to omit lines in the middle to attempt to preserve natural "block" boundaries
- **Header priority**: lines that introduce a nested block (e.g., `def foo():`) get a small priority boost to ensure they survive tight budgets.
# Algorithm
## Footnotes
- <sup><b>[1]</b></sup> <b>Optimized tree representation</b>: An arena‑style tree stored in flat, contiguous buffers. Each node records its kind and value plus index ranges into shared child and key arrays. Arrays are ingested in a single pass and may be deterministically pre‑sampled: the first element is always kept; additional elements are selected via a fixed per‑index inclusion test; for kept elements, original indices are stored and full lengths are counted. This enables accurate omission info and internal gap markers later, while minimizing pointer chasing.
- <sup><b>[2]</b></sup> <b>Priority order</b>: Nodes are scored so previews surface representative structure and values first. Arrays can favor head/mid/tail coverage (default) or strictly the head; tail preference flips head/tail when configured. Object properties are ordered by key, and strings expand by grapheme with early characters prioritized over very deep expansions.
- <sup><b>[3]</b></sup> <b>Choose top N nodes (binary search)</b>: Iteratively picks N so that the rendered preview fits within the byte budget, looping between “choose N” and a render attempt to converge quickly.
- <sup><b>[4]</b></sup> <b>Render attempt</b>: Serializes the currently included nodes using the selected template. Omission summaries and per-file section headers appear in display templates (pseudo/js); json remains strict. For arrays, display templates may insert internal gap markers between non‑contiguous kept items using original indices.
- <sup><b>[5]</b></sup> <b>Diagram source</b>: The Algorithm diagram is generated from `docs/diagrams/algorithm.mmd`. Regenerate the SVG with `cargo make diagrams` before releasing.
## License
MIT