package attesto

import "context"

// PageFunc fetches one limit/offset page of list results.
type PageFunc func(ctx context.Context, limit, offset int) ([]M, error)

// Iterator walks limit/offset pages of a list endpoint transparently, stopping
// on the first short page. Same endpoints, no new API surface.
type Iterator struct {
	fetch    PageFunc
	pageSize int
	buffer   []M
	offset   int
	done     bool
}

// NewIterator wraps any limit/offset PageFunc. pageSize <= 0 defaults to 100.
func NewIterator(fetch PageFunc, pageSize int) *Iterator {
	if pageSize <= 0 {
		pageSize = 100
	}
	return &Iterator{fetch: fetch, pageSize: pageSize}
}

// Next returns the next item, or (nil, nil) when the listing is exhausted.
func (it *Iterator) Next(ctx context.Context) (M, error) {
	if len(it.buffer) == 0 && !it.done {
		page, err := it.fetch(ctx, it.pageSize, it.offset)
		if err != nil {
			return nil, err
		}
		it.offset += it.pageSize
		if len(page) < it.pageSize {
			it.done = true
		}
		it.buffer = page
	}
	if len(it.buffer) == 0 {
		return nil, nil
	}
	item := it.buffer[0]
	it.buffer = it.buffer[1:]
	return item, nil
}

// IterTenantStreamEvents returns an iterator over a stream's events.
func (c *Client) IterTenantStreamEvents(streamID string, pageSize int) *Iterator {
	return NewIterator(func(ctx context.Context, limit, offset int) ([]M, error) {
		return c.ListTenantStreamEvents(ctx, streamID, limit, offset)
	}, pageSize)
}

// IterTenantWindows returns an iterator over a stream's windows.
func (c *Client) IterTenantWindows(streamID string, pageSize int) *Iterator {
	return NewIterator(func(ctx context.Context, limit, offset int) ([]M, error) {
		return c.ListTenantWindows(ctx, streamID, limit, offset)
	}, pageSize)
}

// IterTenantCheckpoints returns an iterator over a stream's checkpoints.
func (c *Client) IterTenantCheckpoints(streamID string, pageSize int) *Iterator {
	return NewIterator(func(ctx context.Context, limit, offset int) ([]M, error) {
		return c.ListTenantCheckpoints(ctx, streamID, limit, offset)
	}, pageSize)
}

// IterForkEvidence returns an iterator over a stream's fork evidence.
func (c *Client) IterForkEvidence(streamID string, pageSize int) *Iterator {
	return NewIterator(func(ctx context.Context, limit, offset int) ([]M, error) {
		return c.ListForkEvidence(ctx, streamID, limit, offset)
	}, pageSize)
}

// IterTenantIVCEpochs returns an iterator over a stream's IVC epochs.
func (c *Client) IterTenantIVCEpochs(streamID string, pageSize int) *Iterator {
	return NewIterator(func(ctx context.Context, limit, offset int) ([]M, error) {
		return c.ListTenantIVCEpochs(ctx, streamID, limit, offset)
	}, pageSize)
}