//! Phase B, the forms half: [`makeover_layout::Field`] rendered to HTML.
//!
//! # Why this emits strings
//!
//! Both webview apps build their markup as strings and hand it to `innerHTML`:
//! goingson's `renderFormField` returns a template literal that fifteen call
//! sites interpolate into larger literals, and Balanced Breakfast's builds
//! nodes but appends them into the same string-built forms. Returning nodes
//! would rewrite the surrounding templates as well, which makes it a migration
//! rather than an adoption. So: strings, and the escaping comes with them.
//!
//! # Why one escaper is enough here
//!
//! goingson carries four escapers and 543 call sites that must pick between
//! them, because `escapeHtml` is built on `textContent` serialization and
//! **`textContent` refuses to encode `"`**. That is what makes it unsound in an
//! attribute, and it is the whole reason the choice exists. Its `escape.js`
//! records the finding as the CHRONIC-XSS seal, and its test suite has a gate
//! keeping the unsafe one off the namespace.
//!
//! [`escape`] here is not built on that, so it encodes the quote along with
//! everything else, which makes one function sound in both sinks. The four-way
//! choice does not move into Rust: it disappears. Nothing in this module hands
//! an unescaped value to the output except through [`Markup`], which a caller
//! has to name.
//!
//! # What the description does not carry
//!
//! [`Field`] describes the field and not its contents, so three things arrive
//! from the renderer side in [`Filling`]: the current value, the options of a
//! select, and the placeholder. The first two are genuinely renderer state. The
//! third is user-facing text and belongs with `label` and `hint` in
//! makeover-layout; it lives here because that crate is published and adding a
//! field to `Field` is a breaking change, not because this is its home.

use crate::{Emit, class};
use makeover_layout::{Field, FieldKind};
use std::fmt::Write as _;

/// A string that is already markup, and is emitted without escaping.
///
/// The one hole in the escaping, and it has to be named to be used. goingson
/// has two live callers that need it, both passing a recurrence-config block
/// built elsewhere, and both would otherwise have their markup rendered as
/// visible angle brackets. A caller constructing this is stating that the
/// contents are trusted; nothing here can check that for them.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct Markup<'a>(pub &'a str);

/// One option of a select.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct Choice<'a> {
    /// What is submitted.
    pub value: &'a str,
    /// What is read.
    pub label: &'a str,
}

impl<'a> Choice<'a> {
    /// An option whose submitted value is also its label.
    #[must_use]
    pub const fn plain(value: &'a str) -> Self {
        Self {
            value,
            label: value,
        }
    }
}

/// What the field currently holds.
///
/// An enum rather than a bag of optional fields, on the same reasoning
/// [`makeover_layout::Depth`] is one: a select with no options and a checkbox
/// with a string value are both unsayable here, where a struct would let them
/// be said and then have to cope.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
pub enum Value<'a> {
    /// Nothing yet.
    #[default]
    Absent,
    /// The value of anything that takes typed text.
    Text(&'a str),
    /// The options of a select, and which of them is current.
    Chosen {
        /// Every option, in the order they are offered.
        options: &'a [Choice<'a>],
        /// The current value. Matched against each option's `value`.
        value: &'a str,
    },
    /// A checkbox, on or off.
    On(bool),
}

impl<'a> Value<'a> {
    /// The value as text, for the kinds that submit one.
    const fn as_text(&self) -> &'a str {
        match self {
            Self::Text(text) | Self::Chosen { value: text, .. } => text,
            Self::Absent | Self::On(_) => "",
        }
    }
}

/// Everything about the field that the description does not carry.
#[derive(Debug, Clone, Copy, Default)]
pub struct Filling<'a> {
    /// What the field holds now.
    pub value: Value<'a>,
    /// Ghost text shown while the field is empty.
    pub placeholder: Option<&'a str>,
    /// Markup appended inside the group, after the hint. Not escaped.
    pub trailing: Option<Markup<'a>>,
    /// Scopes the `id` attributes to one instance of the form.
    ///
    /// The field's `name` is what the value submits under and is the same
    /// wherever the form appears; its `id` has to be unique in the document,
    /// and those two facts stop agreeing the moment a form appears twice.
    /// goingson hits this directly: its new-task and edit-task modals are the
    /// same field set, so it prefixes `form-modal-task-new` or `-edit` to keep
    /// `label for` and `aria-describedby` pointing at the right control.
    ///
    /// Applies to `id`, `for` and the `-hint` / `-error` associations. Never to
    /// `name`, which would change what the form submits.
    pub id_prefix: Option<&'a str>,
}

impl<'a> Filling<'a> {
    /// A filling that carries a value and nothing else.
    #[must_use]
    pub const fn of(value: Value<'a>) -> Self {
        Self {
            value,
            placeholder: None,
            trailing: None,
            id_prefix: None,
        }
    }

    /// The document-unique id for a field of this name.
    fn id_for(&self, name: &str) -> String {
        match self.id_prefix {
            Some(prefix) => format!("{}-{}", escape(prefix), escape(name)),
            None => escape(name),
        }
    }
}

/// Encode the five characters that let a value stop being a value.
///
/// Sound in element text and in a double-quoted attribute alike, which is the
/// property `textContent`-based escaping cannot have. Both sinks are covered by
/// one function so that no call site has to choose, here or downstream.
#[must_use]
pub fn escape(text: &str) -> String {
    let mut out = String::with_capacity(text.len());
    for ch in text.chars() {
        match ch {
            '&' => out.push_str("&amp;"),
            '<' => out.push_str("&lt;"),
            '>' => out.push_str("&gt;"),
            '"' => out.push_str("&quot;"),
            '\'' => out.push_str("&#39;"),
            other => out.push(other),
        }
    }
    out
}

/// The `type` an input takes for a kind.
///
/// [`FieldKind::Secret`] is `password`, which both apps already map by hand.
const fn input_type(kind: FieldKind) -> &'static str {
    match kind {
        FieldKind::Secret => "password",
        FieldKind::Number => "number",
        FieldKind::Checkbox => "checkbox",
        FieldKind::Hidden => "hidden",
        // Select and Textarea are not inputs at all; they never reach here.
        FieldKind::Text | FieldKind::Select | FieldKind::Textarea => "text",
    }
}

/// The attributes every visible control carries, error state included.
///
/// `aria-invalid` is the whole reason the error state is readable at all: the
/// generated stylesheet keys the danger ring on `[aria-invalid="true"]` rather
/// than on a class, so a control rendered already-invalid without it is styled
/// as if nothing were wrong. goingson's runtime validation path sets the
/// attribute and its initial render does not, which is exactly the drift one
/// emitter removes.
/// `id` and `name` arrive separately because they are not the same fact. The
/// name is what submits and is fixed by the description; the id has to be
/// unique in the document and so carries [`Filling::id_prefix`] when a form
/// appears more than once.
fn control_attributes(field: &Field<'_>, id: &str, name: &str) -> String {
    let mut attrs = format!(" id=\"{id}\" name=\"{}\"", escape(name));
    if field.required {
        attrs.push_str(" required");
    }
    if field.invalid() {
        attrs.push_str(" aria-invalid=\"true\"");
    }

    // Both associations, in the order they are useful: the standing help, then
    // what is currently wrong. goingson's runtime path points describedby at
    // the error alone and drops the hint association it never made in the first
    // place; naming both here means the hint survives an error appearing.
    let mut described = Vec::new();
    if field.hint.is_some() {
        described.push(format!("{id}-hint"));
    }
    if field.error.is_some() {
        described.push(format!("{id}-error"));
    }
    if !described.is_empty() {
        let _ = write!(attrs, " aria-describedby=\"{}\"", described.join(" "));
    }
    attrs
}

/// The options of a select, with an unmatched current value carried as its own.
///
/// A select handed a value no option carries renders with nothing selected, the
/// browser falls back to the first option, and the next save writes a value
/// nobody chose. goingson hit exactly that with a backup-retention default of
/// 10 against a 1/3/7/14/0 list, and grew this stray-option fix locally; it is
/// here so the second app gets it without hitting the bug first.
fn options_html(options: &[Choice<'_>], value: &str) -> String {
    let mut html = String::new();
    if !value.is_empty() && !options.iter().any(|opt| opt.value == value) {
        let escaped = escape(value);
        let _ = write!(
            html,
            "<option value=\"{escaped}\" selected data-unmatched=\"true\">{escaped}</option>"
        );
    }
    for opt in options {
        let selected = if opt.value == value { " selected" } else { "" };
        let _ = write!(
            html,
            "<option value=\"{}\"{selected}>{}</option>",
            escape(opt.value),
            escape(opt.label)
        );
    }
    html
}

/// The control itself, without its label, hint or error.
fn control_html(field: &Field<'_>, filling: &Filling<'_>, opts: &Emit) -> String {
    let id = filling.id_for(field.name);
    let attrs = control_attributes(field, &id, field.name);
    let field_class = class("field", opts);
    let placeholder = filling.placeholder.map_or_else(String::new, |text| {
        format!(" placeholder=\"{}\"", escape(text))
    });

    match field.kind {
        FieldKind::Textarea => format!(
            "<textarea class=\"{field_class}\"{attrs}{placeholder}>{}</textarea>",
            escape(filling.value.as_text())
        ),
        FieldKind::Select => {
            let options = match filling.value {
                Value::Chosen { options, value } => options_html(options, value),
                // Described as a select and filled as something else. Emitting
                // an empty select says so on screen rather than in a log.
                _ => String::new(),
            };
            format!("<select class=\"{field_class}\"{attrs}>{options}</select>")
        }
        FieldKind::Checkbox => {
            let checked = if matches!(filling.value, Value::On(true)) {
                " checked"
            } else {
                ""
            };
            format!(
                "<label class=\"{}\"><input type=\"checkbox\"{attrs}{checked}><span>{}</span></label>",
                class("form-checkbox-label", opts),
                escape(field.label)
            )
        }
        // A secret never carries its value into the markup. `FieldKind::secret`
        // is documented as a value that must not be round-tripped through
        // anything that might persist it, and the DOM is such a thing: it is
        // read by every extension on the page and is the first thing a crash
        // reporter serialises. Neither app pre-fills one today, so this costs
        // nothing and closes the door before something does.
        FieldKind::Secret => format!(
            "<input type=\"password\" class=\"{field_class}\"{attrs}{placeholder}>"
        ),
        kind => format!(
            "<input type=\"{}\" class=\"{field_class}\"{attrs}{placeholder} value=\"{}\">",
            input_type(kind),
            escape(filling.value.as_text())
        ),
    }
}

/// One field, as the group the app drops into its form.
///
/// The shape is goingson's, down to the class names, so adoption there deletes
/// `renderFormField` rather than restyling anything. That is also why the class
/// names are not emitted by [`crate::stylesheet`]: `.form-group`, `.form-label`,
/// `.form-hint` and `.form-error` are the apps' own, and phase A deliberately
/// emits only what it can generate from the description. Whether they should
/// move into the description is the next question this raises, not one it
/// answers.
///
/// A [`FieldKind::Hidden`] field is the input alone: no group, no label, and
/// nothing drawn, which is what [`FieldKind::visible`] means.
///
/// The error marks the group as well as the control. That is
/// [`Field::invalid`]'s own reasoning: a renderer with no descendant selectors
/// cannot find the group from the message, so the group has to be told.
///
/// ```
/// use makeover_layout::{Field, FieldKind};
/// use makeover_webview::{Emit, form::{Filling, Value, field_html}};
///
/// let field = Field::new(FieldKind::Text, "title", "Title");
/// let html = field_html(&field, &Filling::of(Value::Text("Ship it")), &Emit::default());
///
/// assert!(html.contains(r#"<label class="form-label" for="title">Title</label>"#));
/// assert!(html.contains(r#"value="Ship it""#));
/// ```
#[must_use]
pub fn field_html(field: &Field<'_>, filling: &Filling<'_>, opts: &Emit) -> String {
    let id = filling.id_for(field.name);

    if !field.kind.visible() {
        // Name only, no id: a hidden field is never pointed at by a label or a
        // description, so the one attribute it needs is the one that submits.
        return format!(
            "<input type=\"hidden\" name=\"{}\" value=\"{}\">",
            escape(field.name),
            escape(filling.value.as_text())
        );
    }

    let mut html = format!("<div class=\"{}", class("form-group", opts));
    if field.invalid() {
        html.push_str(" has-error");
    }
    if field.extended {
        // The disclosure that hides these is a property of the form, not of the
        // field, so the field is marked and the app opens or closes the group.
        html.push_str("\" data-extended=\"true");
    }
    html.push_str("\">");

    // A checkbox labels itself, on the right of the box. Both apps special-case
    // this inline today, which is the tell that it belongs in the description;
    // `FieldKind::labels_itself` is where it went.
    if !field.kind.labels_itself() {
        let _ = write!(
            html,
            "<label class=\"{}\" for=\"{id}\">{}</label>",
            class("form-label", opts),
            escape(field.label)
        );
    }

    html.push_str(&control_html(field, filling, opts));

    if let Some(hint) = field.hint {
        let _ = write!(
            html,
            "<div class=\"{}\" id=\"{id}-hint\">{}</div>",
            class("form-hint", opts),
            escape(hint)
        );
    }
    if let Some(Markup(markup)) = filling.trailing {
        html.push_str(markup);
    }
    if let Some(error) = field.error {
        let _ = write!(
            html,
            "<div class=\"{} visible\" id=\"{id}-error\" role=\"alert\">{}</div>",
            class("form-error", opts),
            escape(error)
        );
    }

    html.push_str("</div>");
    html
}

#[cfg(test)]
mod tests {
    use super::*;

    fn field(kind: FieldKind) -> Field<'static> {
        Field::new(kind, "title", "Title")
    }

    #[test]
    fn a_value_cannot_break_out_of_the_attribute_it_sits_in() {
        // The payload from goingson's own CHRONIC-XSS regression test.
        let filling = Filling::of(Value::Text("x\" onfocus=alert(1) autofocus=\""));
        let html = field_html(&field(FieldKind::Text), &filling, &Emit::default());
        // The payload survives as text, which is the point: it is inert
        // because the quote that would have closed the attribute is encoded,
        // not because the words were filtered.
        assert!(!html.contains("\" onfocus"), "{html}");
        assert!(
            html.contains("value=\"x&quot; onfocus=alert(1) autofocus=&quot;\""),
            "{html}"
        );
    }

    #[test]
    fn a_label_cannot_open_a_tag() {
        let mut f = field(FieldKind::Text);
        f.label = "<script>alert(1)</script>";
        let html = field_html(&f, &Filling::default(), &Emit::default());
        assert!(!html.contains("<script>"), "{html}");
        assert!(html.contains("&lt;script&gt;"), "{html}");
    }

    #[test]
    fn every_escaped_sink_is_covered_by_the_one_escaper() {
        assert_eq!(escape("&<>\"'"), "&amp;&lt;&gt;&quot;&#39;");
        // The character `textContent` serialization leaves alone, which is why
        // the app needs two escapers and this needs one.
        assert!(escape("\"").contains("&quot;"));
    }

    #[test]
    fn markup_is_the_only_way_past_the_escaping() {
        let filling = Filling {
            trailing: Some(Markup("<div class=\"recurrence-config\"></div>")),
            ..Filling::default()
        };
        let html = field_html(&field(FieldKind::Text), &filling, &Emit::default());
        assert!(html.contains("<div class=\"recurrence-config\"></div>"), "{html}");
    }

    #[test]
    fn an_invalid_field_carries_the_attribute_its_own_stylesheet_keys_on() {
        let mut f = field(FieldKind::Text);
        f.error = Some("Required");
        let opts = Emit::default();
        let html = field_html(&f, &Filling::default(), &opts);
        assert!(html.contains("aria-invalid=\"true\""), "{html}");
        // The selector the CSS side emits for exactly this state.
        assert!(crate::stylesheet(&opts).contains("[aria-invalid=\"true\"]"));
        // And the group is marked too, which a renderer without descendant
        // selectors depends on.
        assert!(html.contains("has-error"), "{html}");
    }

    #[test]
    fn a_valid_field_claims_nothing_about_being_invalid() {
        let html = field_html(&field(FieldKind::Text), &Filling::default(), &Emit::default());
        assert!(!html.contains("aria-invalid"), "{html}");
        assert!(!html.contains("has-error"), "{html}");
    }

    #[test]
    fn the_hint_survives_an_error_arriving() {
        let mut f = field(FieldKind::Text);
        f.hint = Some("Keep it short");
        f.error = Some("Required");
        let html = field_html(&f, &Filling::default(), &Emit::default());
        assert!(
            html.contains("aria-describedby=\"title-hint title-error\""),
            "{html}"
        );
    }

    #[test]
    fn a_secret_never_carries_its_value_into_the_markup() {
        let filling = Filling::of(Value::Text("hunter2"));
        let html = field_html(&field(FieldKind::Secret), &filling, &Emit::default());
        assert!(!html.contains("hunter2"), "{html}");
        assert!(html.contains("type=\"password\""), "{html}");
    }

    #[test]
    fn a_hidden_field_is_the_input_and_nothing_else() {
        let filling = Filling::of(Value::Text("42"));
        let html = field_html(&field(FieldKind::Hidden), &filling, &Emit::default());
        assert_eq!(html, "<input type=\"hidden\" name=\"title\" value=\"42\">");
    }

    #[test]
    fn a_checkbox_labels_itself_and_takes_no_separate_label() {
        let html = field_html(
            &field(FieldKind::Checkbox),
            &Filling::of(Value::On(true)),
            &Emit::default(),
        );
        assert!(!html.contains("form-label"), "{html}");
        assert!(html.contains("checked"), "{html}");
        assert!(html.contains("<span>Title</span>"), "{html}");
    }

    #[test]
    fn a_select_keeps_a_value_no_option_carries() {
        let options = [Choice::plain("1"), Choice::plain("3"), Choice::plain("7")];
        let filling = Filling::of(Value::Chosen {
            options: &options,
            value: "10",
        });
        let html = field_html(&field(FieldKind::Select), &filling, &Emit::default());
        assert!(html.contains("data-unmatched=\"true\""), "{html}");
        // Selected, so the next save round-trips it rather than writing the
        // first option over the top of it.
        assert!(html.contains("<option value=\"10\" selected"), "{html}");
    }

    #[test]
    fn a_select_marks_the_option_that_matches() {
        let options = [Choice::plain("1"), Choice::plain("3")];
        let filling = Filling::of(Value::Chosen {
            options: &options,
            value: "3",
        });
        let html = field_html(&field(FieldKind::Select), &filling, &Emit::default());
        assert!(html.contains("<option value=\"3\" selected>3</option>"), "{html}");
        assert!(html.contains("<option value=\"1\">1</option>"), "{html}");
        assert!(!html.contains("data-unmatched"), "{html}");
    }

    #[test]
    fn a_textarea_carries_its_value_as_text_and_not_as_an_attribute() {
        let filling = Filling::of(Value::Text("two\nlines"));
        let html = field_html(&field(FieldKind::Textarea), &filling, &Emit::default());
        assert!(html.contains(">two\nlines</textarea>"), "{html}");
    }

    #[test]
    fn the_class_prefix_reaches_the_markup_as_well_as_the_stylesheet() {
        let opts = Emit {
            class_prefix: "mk-",
            ..Emit::default()
        };
        let html = field_html(&field(FieldKind::Text), &Filling::default(), &opts);
        assert!(html.contains("class=\"mk-form-group\""), "{html}");
        assert!(html.contains("class=\"mk-field\""), "{html}");
    }

    #[test]
    fn an_extended_field_says_so_and_leaves_the_disclosure_to_the_form() {
        let mut f = field(FieldKind::Text);
        f.extended = true;
        let html = field_html(&f, &Filling::default(), &Emit::default());
        assert!(html.contains("data-extended=\"true\""), "{html}");
    }

    /// The prefix scopes the id and leaves the name alone. Prefixing the name
    /// too would change what the form submits, which is the failure this pair
    /// of assertions exists to catch rather than describe.
    #[test]
    fn the_id_prefix_scopes_the_id_and_never_the_name() {
        let mut f = field(FieldKind::Text);
        f.hint = Some("Keep it short");
        f.error = Some("Required");
        let filling = Filling {
            id_prefix: Some("form-modal-task-edit"),
            ..Filling::default()
        };
        let html = field_html(&f, &filling, &Emit::default());

        assert!(html.contains(r#"id="form-modal-task-edit-title""#), "{html}");
        assert!(html.contains(r#"name="title""#), "{html}");
        assert!(!html.contains(r#"name="form-modal-task-edit-title""#), "{html}");

        // The label and both associations follow the id, or they point at
        // nothing once the same form is on screen twice.
        assert!(
            html.contains(r#"for="form-modal-task-edit-title""#),
            "{html}"
        );
        assert!(
            html.contains(
                r#"aria-describedby="form-modal-task-edit-title-hint form-modal-task-edit-title-error""#
            ),
            "{html}"
        );
        assert!(
            html.contains(r#"id="form-modal-task-edit-title-hint""#),
            "{html}"
        );
    }

    #[test]
    fn a_hidden_field_submits_its_bare_name_under_a_prefix() {
        let filling = Filling {
            value: Value::Text("42"),
            id_prefix: Some("scoped"),
            ..Filling::default()
        };
        let html = field_html(&field(FieldKind::Hidden), &filling, &Emit::default());
        assert_eq!(html, r#"<input type="hidden" name="title" value="42">"#);
    }

    #[test]
    fn no_prefix_leaves_the_id_as_the_name() {
        let html = field_html(&field(FieldKind::Text), &Filling::default(), &Emit::default());
        assert!(html.contains(r#"id="title" name="title""#), "{html}");
    }
}