//! TRACES: FR-CAT-10 | NFR-PORT-1
//! Finding the card.
//!
//! FR-CAT-10 asks for removable-volume insertion to be detected "where the
//! platform permits", which is a careful phrase and this module is why. There
//! is no portable answer: Linux has a mount table and a sysfs flag, Android
//! has neither and hands out a document tree the user picked (ARCH §6.9).
//! So this reports what it can and returns an empty list where it cannot,
//! and every caller must still offer the user a way to say where the card is.
//!
//! # This is the one place besides `grant` that names a path
//!
//! `storage` explains why nothing above it takes a `Path`: a library location
//! is a [`crate::storage::LocalStorage::grant`] away from being a `RootId`,
//! and Android has no path to give. Volume discovery sits on the same side of
//! that line — it produces exactly what `grant` consumes, the folder that is
//! about to become a root. It is discovery of a path rather than use of one.
//!
//! # Why a heuristic rather than a device manager
//!
//! Talking to udisks2 over D-Bus would be authoritative and would add a D-Bus
//! dependency, a service that may not be running, and a permission dialog on
//! some desktops — for a question the filesystem can answer directly. The
//! mount table plus the sysfs `removable` flag covers USB card readers,
//! phones mounted as storage, and external drives. What it does not cover is
//! an internal SD reader that reports itself fixed, which is why a `DCIM`
//! directory is reported alongside and weighted more heavily than the flag:
//! a volume with `DCIM` on it is a camera card whatever sysfs believes.

use std::path::PathBuf;
// Only the mount-table reader below borrows a path; `Volume` owns one.
#[cfg(target_os = "linux")]
use std::path::Path;

/// Somewhere a card might be.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Volume {
    /// What to call it in a list. The mount point's last component, which is
    /// what desktops name after the filesystem label.
    pub label: String,
    /// Where it is mounted. Hand this to
    /// [`LocalStorage::grant`](crate::storage::LocalStorage::grant).
    pub path: PathBuf,
    /// Whether the kernel calls the underlying device removable.
    pub removable: bool,
    /// Whether it holds a `DCIM` directory — the strongest signal there is
    /// that this is a camera card rather than a backup drive.
    pub has_dcim: bool,
}

impl Volume {
    /// Whether to show this one first.
    ///
    /// `DCIM` outranks the kernel flag: an internal SD reader often reports
    /// its device as fixed, and the user with a card in it does not care what
    /// sysfs thinks.
    pub fn is_likely_card(&self) -> bool {
        self.has_dcim || self.removable
    }
}

/// TRACES: FR-CAT-10 | NFR-PORT-1
/// Whether an import can reach a card on this platform at all.
///
/// **Not the same question as "did [`volumes`] find anything".** An empty list
/// on Linux means no card is plugged in, and the user can plug one in or type
/// where it is mounted. `false` here means the operation cannot be performed
/// however hard the user tries, and the interface should not offer it.
///
/// It is `false` on Android, for two reasons that both have to be fixed before
/// it can change:
///
/// - There is no mount table to read and no path to type. Storage is reached
///   through a tree the user granted, and a removable volume appears there or
///   not at all (ARCH §6.9).
/// - Nothing implements [`WritableStorage`](crate::WritableStorage) except
///   [`LocalStorage`](crate::LocalStorage), so there is no destination to write
///   into even once a source is named.
///
/// The engine above this is already portable — it takes storage traits and
/// never a path — so what this gates is the *interface*, and it stops being
/// `false` when a SAF implementation lands rather than when the importer is
/// rewritten.
pub const fn imports_supported() -> bool {
    cfg!(target_os = "linux")
}

/// Every mounted volume that might hold photographs.
///
/// Ordered with the likely cards first and, within each group, by label, so
/// the list does not reorder itself between two calls a second apart.
/// Returns empty where the platform does not permit the question — which is
/// not an error and must not be presented as one.
pub fn volumes() -> Vec<Volume> {
    let mut found = platform_volumes();
    found.sort_by(|a, b| {
        b.is_likely_card()
            .cmp(&a.is_likely_card())
            .then_with(|| a.label.cmp(&b.label))
    });
    found
}

#[cfg(target_os = "linux")]
fn platform_volumes() -> Vec<Volume> {
    let table = match std::fs::read_to_string("/proc/mounts") {
        Ok(t) => t,
        Err(e) => {
            log::debug!("no mount table: {e}");
            return Vec::new();
        }
    };
    parse_mounts(&table)
        .into_iter()
        .filter(is_candidate)
        .map(|m| {
            let removable = device_is_removable(&m.device).unwrap_or(false)
                // A desktop that automounts under /run/media or /media has
                // already decided this is removable, and it had udisks2 to ask.
                || under_media_dir(&m.mount_point);
            Volume {
                label: label_for(&m.mount_point),
                has_dcim: m.mount_point.join("DCIM").is_dir(),
                removable,
                path: m.mount_point,
            }
        })
        .collect()
}

/// Everywhere else: no answer, and saying so is the honest result.
///
/// On Android the question is not merely unanswerable but wrong — storage is
/// reached through a tree the user granted, and a card appears there or not at
/// all (ARCH §6.9, FR-PLAT-AND-1).
#[cfg(not(target_os = "linux"))]
fn platform_volumes() -> Vec<Volume> {
    Vec::new()
}

/// One line of the mount table.
#[cfg(target_os = "linux")]
#[derive(Debug, Clone, PartialEq, Eq)]
struct Mount {
    device: String,
    mount_point: PathBuf,
    fs_type: String,
}

/// Parse `/proc/mounts`.
///
/// Split out from the reading so it can be tested against real tables without
/// a card plugged in — which is the only way this file is testable at all.
#[cfg(target_os = "linux")]
fn parse_mounts(table: &str) -> Vec<Mount> {
    table
        .lines()
        .filter_map(|line| {
            let mut fields = line.split_whitespace();
            let device = fields.next()?;
            let mount_point = fields.next()?;
            let fs_type = fields.next()?;
            Some(Mount {
                device: unescape(device),
                mount_point: PathBuf::from(unescape(mount_point)),
                fs_type: fs_type.to_string(),
            })
        })
        .collect()
}

/// Undo the octal escaping the kernel applies to spaces and tabs.
///
/// A card is very often labelled with a space in it — `EOS DIGITAL` is what
/// Canon writes — and mounted at `/run/media/duncan/EOS\040DIGITAL`. Without
/// this the path does not exist and the card is invisible.
#[cfg(target_os = "linux")]
fn unescape(field: &str) -> String {
    let mut out = String::with_capacity(field.len());
    let bytes = field.as_bytes();
    let mut i = 0;
    while i < bytes.len() {
        if bytes[i] == b'\\' && i + 3 < bytes.len() {
            let digits = &field[i + 1..i + 4];
            if let Ok(c) = u8::from_str_radix(digits, 8) {
                out.push(c as char);
                i += 4;
                continue;
            }
        }
        out.push(bytes[i] as char);
        i += 1;
    }
    out
}

/// Whether a mount could hold a photograph library at all.
///
/// Excludes the pseudo-filesystems, which is most of the table, and anything
/// not backed by a block device. A card reader always is.
#[cfg(target_os = "linux")]
fn is_candidate(m: &Mount) -> bool {
    const FILESYSTEMS: &[&str] = &[
        // What cameras format cards as, in practice: FAT32 below 32 GB,
        // exFAT above it, which is why an SDXC card is always exfat.
        "vfat", "exfat", "ntfs", "ntfs3",
        "fuseblk", // And what an external drive carrying an archive is.
        "ext2", "ext3", "ext4", "btrfs", "xfs", "f2fs", "hfsplus", "apfs",
    ];
    m.device.starts_with("/dev/") && FILESYSTEMS.contains(&m.fs_type.as_str())
}

/// Whether the desktop automounted this, which means it already decided.
#[cfg(target_os = "linux")]
fn under_media_dir(mount_point: &Path) -> bool {
    let s = mount_point.to_string_lossy();
    s.starts_with("/run/media/") || s.starts_with("/media/") || s.starts_with("/mnt/media/")
}

/// Ask sysfs whether the device behind a partition is removable.
///
/// `/dev/sdb1` is a partition; the flag lives on its whole-disk parent,
/// `/sys/block/sdb/removable`. Trailing digits are stripped to get there,
/// except on `mmcblk0p1` and `nvme0n1p1`, whose parents keep their digits and
/// lose only the `pN` suffix — strip naively and `mmcblk0p1` becomes
/// `mmcblk`, which does not exist, and every SD card in an internal reader
/// reports itself fixed.
#[cfg(target_os = "linux")]
fn device_is_removable(device: &str) -> Option<bool> {
    let name = device.strip_prefix("/dev/")?;
    let disk = whole_disk(name);
    let flag = std::fs::read_to_string(format!("/sys/block/{disk}/removable")).ok()?;
    Some(flag.trim() == "1")
}

/// The whole-disk name a partition belongs to.
#[cfg(target_os = "linux")]
fn whole_disk(partition: &str) -> String {
    // `mmcblk0p1` -> `mmcblk0`, `nvme0n1p3` -> `nvme0n1`: these spell the
    // partition with a `p`, and the disk name legitimately ends in a digit.
    if partition.starts_with("mmcblk") || partition.starts_with("nvme") {
        if let Some((disk, tail)) = partition.rsplit_once('p') {
            if !tail.is_empty() && tail.chars().all(|c| c.is_ascii_digit()) {
                return disk.to_string();
            }
        }
        return partition.to_string();
    }
    // `sdb1` -> `sdb`. A SCSI-style disk name never ends in a digit.
    partition
        .trim_end_matches(|c: char| c.is_ascii_digit())
        .to_string()
}

/// What to call a volume in a list.
#[cfg(target_os = "linux")]
fn label_for(mount_point: &Path) -> String {
    mount_point
        .file_name()
        .map(|n| n.to_string_lossy().to_string())
        .unwrap_or_else(|| mount_point.to_string_lossy().to_string())
}

// Reading `/proc/mounts` and naming block devices is what these test, so
// they are as Linux-bound as the code above.
#[cfg(all(test, target_os = "linux"))]
mod tests {
    use super::*;

    const TABLE: &str = "\
proc /proc proc rw,nosuid 0 0
sys /sys sysfs rw,nosuid 0 0
/dev/nvme0n1p2 / btrfs rw,noatime 0 0
/dev/nvme0n1p1 /boot vfat rw,relatime 0 0
/dev/sdb1 /run/media/duncan/EOS\\040DIGITAL exfat rw,nosuid 0 0
tmpfs /run/user/1000 tmpfs rw,nosuid 0 0
";

    #[test]
    fn a_mount_table_yields_its_block_devices() {
        let mounts = parse_mounts(TABLE);
        assert_eq!(mounts.len(), 6);
        let candidates: Vec<_> = mounts.iter().filter(|m| is_candidate(m)).collect();
        // Three block-backed filesystems; the pseudo ones are gone.
        assert_eq!(candidates.len(), 3);
        assert!(candidates.iter().all(|m| m.device.starts_with("/dev/")));
    }

    #[test]
    fn a_card_labelled_with_a_space_is_not_lost() {
        // Canon writes `EOS DIGITAL`, the kernel escapes the space, and a path
        // that keeps the escape does not exist.
        let mounts = parse_mounts(TABLE);
        let card = mounts
            .iter()
            .find(|m| m.device == "/dev/sdb1")
            .expect("the card");
        assert_eq!(
            card.mount_point,
            PathBuf::from("/run/media/duncan/EOS DIGITAL")
        );
        assert_eq!(label_for(&card.mount_point), "EOS DIGITAL");
    }

    #[test]
    fn an_automounted_volume_is_treated_as_removable() {
        // The desktop had udisks2 to ask and already decided.
        assert!(under_media_dir(Path::new("/run/media/duncan/EOS DIGITAL")));
        assert!(under_media_dir(Path::new("/media/usb0")));
        assert!(!under_media_dir(Path::new("/home/duncan/Photos")));
        // Not a prefix match on the word: `/media-server` is not automounted.
        assert!(!under_media_dir(Path::new("/mediaserver/x")));
    }

    #[test]
    fn a_partition_resolves_to_the_disk_that_carries_the_flag() {
        assert_eq!(whole_disk("sdb1"), "sdb");
        assert_eq!(whole_disk("sda12"), "sda");
        // The naive strip turns these into `mmcblk` and `nvme`, neither of
        // which exists — and every SD card in an internal reader then reports
        // itself fixed.
        assert_eq!(whole_disk("mmcblk0p1"), "mmcblk0");
        assert_eq!(whole_disk("nvme0n1p3"), "nvme0n1");
        // A whole disk named directly is already the answer.
        assert_eq!(whole_disk("mmcblk0"), "mmcblk0");
        assert_eq!(whole_disk("sdb"), "sdb");
    }

    #[test]
    fn a_camera_card_outranks_a_backup_drive() {
        let card = Volume {
            label: "EOS DIGITAL".into(),
            path: "/run/media/duncan/EOS DIGITAL".into(),
            removable: false,
            has_dcim: true,
        };
        let drive = Volume {
            label: "archive".into(),
            path: "/run/media/duncan/archive".into(),
            removable: true,
            has_dcim: false,
        };
        let fixed = Volume {
            label: "boot".into(),
            path: "/boot".into(),
            removable: false,
            has_dcim: false,
        };
        // An internal reader reports its device fixed, and the user with a
        // card in it does not care what sysfs thinks.
        assert!(card.is_likely_card());
        assert!(drive.is_likely_card());
        assert!(!fixed.is_likely_card());
    }

    #[test]
    fn a_platform_that_cannot_import_says_so_separately_from_finding_nothing() {
        // The two are different claims: an empty list means "plug one in",
        // `false` here means "this cannot be done here". An interface that
        // conflated them would offer a page that can never be used.
        assert_eq!(imports_supported(), cfg!(target_os = "linux"));
        if !imports_supported() {
            assert!(volumes().is_empty());
        }
    }

    #[test]
    fn asking_where_there_is_no_answer_is_not_an_error() {
        // The call must work on a machine with no card, in CI, and on a
        // platform that cannot answer at all — an empty list, never a panic.
        let _ = volumes();
    }

    #[test]
    fn an_octal_escape_that_is_not_one_is_left_alone() {
        // A backslash near the end of a field must not read past it.
        assert_eq!(unescape("/mnt/odd\\"), "/mnt/odd\\");
        assert_eq!(unescape("/mnt/a\\04"), "/mnt/a\\04");
        assert_eq!(unescape("/mnt/a\\040b"), "/mnt/a b");
    }
}